Round Ball? Minnie? Bullet? Twist rate? Help.

Superhouse 15 · March 20, 2012, 07:56 AM

Ok, I got my second black powder firearm, this one a .50cal inline percussion. At first it wasn't reliable firing the cap, but a little file work on the nipple fixed that. In the junk collected in the powder cabinet at the gunshop I found Minnie bullets and balls that fit, as well as some solid bullets that resemble the Remington "Buckhammer" slugs, but with wide grease grooves. The local D ick's sporting goods has sabot bullets in various weights. During test firing I loaded FFFG powder at 40gr behind a Minnie lubed with "Bore Butter" and it stabilized at about 20 yards and hit a paper plate. For convenience I made some paper cartridges (that worked... until I bit the end off like the guy on Youtube did, only I got some tasty black powder in my mouth). Development of those will continue.

Anyway, now that it's reliable, does anybody know what style of projectile is correct with a given twist rate? Is there a chart somewhere or a rule of thumb? Anybody have any advice on powder weight? I do know two things, one: Minnie bullets need a certain minimum powder charge to expand and grip the rifling, and two: I don't want to blow my face up. Is there a projectile that works well in any barrel? It has an old but well kept Weaver scope on it, 4X, and I want to be able to shoot it at at least 100yds to take advantage of that. For hunting it's probably realistically going to be shot at 30 to 40yds max.

And while I'm overloading everyone with questions, What about the pre-formed powder pellets? Some even come in a kit with sabot bullets and pellets of powder. Does anybody know how those taste?

Thank you kind black powder Gurus.

mikthestick · March 20, 2012, 08:23 AM

The formula you want for twist is Greenhills formula I can give you it in a later post if you want it. For the moment so as not to confuse you. Lyman BP hand book say a 22" 1 in 24" twist barreled 50 cal Buffalo HP conical of 350grs @1148ft/sec with 60grs of 2f Geox behind it. The twist on barrels designed for balls is 1 in 48. The largest bullet on the chart is 490gr anything less than that should stabalise lovely in a 1 in 24" barrel. 120gr Geox gives 1551 ft/sec. Tell me your powder bullet/ball weight and barrel length and I'll print more info.
A round ball requires less twist to stabalise than a conical bullet If your barrel is 1in 48 twist it will stabalise round balls a conical bullet is required to make the bullet heavier if you made one 200grs(not very conical) it would be much less stable than the round ball. It would almost certainly not stabalise. There is no chance a 350gr conical will stabalise in a 1 in 48" twist barrel
I love maths and formulas but hesitate to print them as not everyone feels as I do. However if you want the info I'll help as much as I can.
mik

Rifleman1776 · March 20, 2012, 08:58 AM

Quote:

A round ball requires less twist to stabalise than a conical bullet If your barrel is 1in 24 twist it will stabalise round balls a conical bullet is required to make the bullet heavier if you made one 200grs(not very conical) it would be much less stable than the round ball. It would almost certainly not stabalise. There is no chance a 350gr conical will stabalise in a 1 in 24" twist barrel

If that statement made any sense it would be easier to reply to.
As a TRADITIONAL muzzle loader with 40 years experience, I (and many others) can tell you the slower twists (e.g. 1:48 to 1:72, or even slower) are better suited for round balls. Elongated bullets stabilize with faster twists (faster than 1:48). Contact the maker of your inline or look on the internet for information.
Start with moderate loads and work up to find what works best. Shooting is the only way to dial in your rifle.
BTW, if you are truly worried your rifle might blow up, hang on wall and do not shoot.

mikthestick · March 20, 2012, 09:05 AM

Please read my post again I have edited out my mistakes. My knowledge of ballistics is far superior to my command of the English language.

**Mike Irwin** · March 20, 2012, 09:26 AM

Most period round ball guns I've examined have twist rates of 1 in 50 or slower. One I saw years ago had a twist of 1 in 90.

Thompson Center for years adopted a compromise twist rate in its guns of about 1 in 48. They would adequately stabilize both round balls and conicals.

When Pennsylvania mandated round ball only, they came out with the Pennsylvania Hunter, which was a .50 with a twist rate of 1 in 66.

The general rule of thumb is the shorter the bullet, the slower the twist rate needed to stabilize it.

The longer the bullet (and heavier, generally) the faster the twist rate.

Hawg · March 20, 2012, 09:37 AM

What Mike said. A 1:48 is a good medium ground for round balls and conicals but not ideal for either.

mikthestick · March 20, 2012, 10:11 AM

The rifle twists I mentioned are from Lyman BP reloading manual. If you buy a rifle how can you check its twist rate. I would have to take it to a gun smith.

Beagle333 · March 20, 2012, 10:27 AM

(Works the same on ML's)

http://www.gunnersden.com/index.htm....ist-rates.html

Hawg · March 20, 2012, 10:39 AM

Going by Beagles link. With a slow twist you're not going to have enough barrel length to make a full turn. I like to push mine all the way down to start with but it doesn't matter. Just mark it at the muzzle in line with the front sight. Push it in or pull it out whichever way you prefer until you get a half turn. Mark it again. Measure between the marks and multiply X2. Or you can go a 1/4 turn and multiply X4

I can't believe I said divide originally. Multiply X2 not divide as I originally stated.

ScottRiqui · March 20, 2012, 10:52 AM

Keep in mind that Greenhill's Twist Rate Formula isn't applicable to round projectiles, and it's really not ideal for any projectile that isn't at least 2.5 or 3 times longer than it is wide. You're better off just going with general guidelines and TLAR (That Looks About Right) than you are trying to justify a particular twist rate by mis-using Greenhill's formula.

Superhouse 15 · March 20, 2012, 11:09 AM

Wow, thanks a lot. I am a gunsmith and as soon as I get to the shop I'll measure the twist rate. I loaded 40gr of FFFG in a brass measure behind the round ball and the Minnie. I also tried a load of birdshot strictly for testing purposes with 45gr but it was dramatically unsuccessful, not surprising considering the rifled barrel and small charge of shot. Bring on the math, please. My first black powder gun was a Spanish Deringer that I was finally able to safely fire with a patched ball and 25gr of the same powder. Still not exactly sure of an appropriate charge weight but I was cautious.

B.L.E. · March 20, 2012, 08:04 PM

Forty grains is a mighty light load in a .50 rifle. Accuracy suffers if a load is too light, not just for Minie balls but also with patched roundballs.
Most people put around 70 to 90 grains in a .50 when shooting patched round balls, even heavier sometimes with the slow twist barrels.

**Mike Irwin** · March 20, 2012, 09:06 PM

My Thompson-Center Renegade .50 flintlock wouldn't shoot well at all with anything less than 60 grains of FFFg.

Hawg · March 21, 2012, 12:08 AM

I used 90 grains in my .50. Oddly enough I use 90 in my .54 also.

deerslayer303 · March 21, 2012, 04:18 AM

40 grains???? Did the ball even make it to the target LOL

. I'm just playing, but yea you need to put some FIRE to that rifle. 90 grains seems like a sweet spot in my .54.

Superhouse 15 · March 21, 2012, 06:57 AM

I was certainly taking a cautious approach to the first few shots. And I did mis type. The powder is FFFFG (four F's). I'll give it a boost and try starting at 70 and working up from there. I'm stuck at work today, going to measure and test fire tomorrow. I believe I'll wait on the Sabot bullets until I see what I can get with the Minnié accuracy wise.

deerslayer303 · March 21, 2012, 07:17 AM

I don't have no where near the experience as some of these guys here. But FFFFg is generally a pan powder for priming flintlocks. Fg is cannon powder, FFg a rifle powder, and FFFg a pistol powder.

Hawg · March 21, 2012, 07:59 AM

Don't use 4F for a main charge. Use 2F or 3F.

Strafer Gott · March 21, 2012, 09:21 AM

I'd stick with the light charges until I knew what gun I had. Some of the legacy inlines are unsafe with major loads. A muzzle loader can be accurate loaded to different levels. If you are shooting for smiles load it light. I would not trust heavy loads without at least remote firing with a proof load. It's only prudent. Try googlin Randy Wakeman- CVA. A cautionary tale- at least something to think about.

Hawg · March 21, 2012, 09:49 AM

Quote:

Try googlin Randy Wakeman- CVA.

Oh PUUHLEEZE!

Fairshake · March 21, 2012, 10:42 AM

I've been shooting BP since 1970 when I was working in a gun store. I purchased two C&B revolvers, one a 36 caliber and the other a 44 cal. I also purchased a 50 Cal Hawken rifle.
The twist in a Hawken is 1-48 which is a middle of the road compromise twist. The best for round ball and patch is in the 1-66 range and slower.
The new inline guns are not meant to fire round ball or the old conical bullets. They shoot the sabot rounds or the big conicals because they have a fast twist.
The best for the conical or the new Hornady Plains bullets is the 1-48 and faster like the 1-36 or so.
2F is the best all around grain size for 50 caliber and above. If using Swiss then the 1 1/2 gets the call.
All black powder is the exact same as far as it's production goes. It's a mixture of three chemicals and the speed at which it burns is determined by the grain size only. It all burns from the outside in and therefore the smaller the grain the faster the burn.
The foreign made powders tend to be smaller than the Goex made in the USA.
I have made several slides and viewed it under a microscope to verify this.
Use your inline with larger charges and the correct componets and you will start to see good groups.
Stay away from 4F unless you are primming a flintlock pan.
3F will burn in your inline but will more than likely not give the better accuracy. Each gun will have it's own favorite load.
Inlines are usually made to fire 209 shotgun primers because of their nature of being harder to ignite the pellet type charges used by many people today.
I have never fired a sub powder and never will as I'm a BP purist.
You should buy one of the books by Sam Fadala which is in it's fith printing. It is named the Complete Blackpowder Handbook.
I disagree with some of his writings but overall it's a good starter book to have. Later David

Pahoo · March 21, 2012, 01:29 PM

Fairshake;
Pretty much sums it up and as he and deerslayer stated, don't use 4F as a main charge. As far as max loads, I see folks loading too hot and aside from potentially being unsafe, it's wasteful as to much powder will wind up on the ground in front of you. ....

Now, I'm still talking In-lines and I never see the need to exceed 90 -95 grns. As far as pellets, your choice but don't care for them and their issues..

Quote:

The best for the conical are the new Hornady Plains bullets

Excellent choice and one of best. if not the best. ...

Quote:

You should buy one of the books by Sam Fadala which is in it's fith printing. It is named the Complete Blackpowder Handbook.

I recommend this book, all the time, to students ...

One thing that I've always thought would be helpful to all, is listing our shot-strings. We all know that range-time is part of this "Great Adventure" and would be nice to share with folks. ...

**Mike Irwin** · March 22, 2012, 06:24 AM

I loaded 135 grains behind a lead round ball in my .50 once.

Once.

The stock didn't fit me all that well and it kicked the absolute crap out of me.

It's a pussycat with 60 to 80 grains, though.

Nathan · March 22, 2012, 07:08 AM

I think it has been hinted at, but I'll say it. 70gr of 4F will likely blow you up.

80 of 2F would be a good place to start behind a sabot or conical of your choice.

An average BP rifle is more than capable taking a deer at 125 yds.

mikthestick · March 22, 2012, 08:36 AM

Sorry I don,t like to write rubbish. My first post was a quote from lyman. What I wrote was BASED ON THE FACT THAT A 1 IN 48 TWIST STABALISES A ROUND BALL OF 50CAL. But that was an ASSUMTION not a fact, don't assume in maths or you risk embarasment

below is the simplest explanation I could fin for Greenhills formula.

150 x diameter squared divided by bullet length = required spin

Example for a .45 caliber bullet .60 inches long:

150 x .45 x .45 divided by .60 = 50.6 inches

So, for the example bullet, a spin rate of 1:50.6 or faster is required

The formula can also provide us with the maximum bullet length which can be stabilized by a given barrel twist. The formula becomes:

150 x diameter squared divided by twist rate

Example for a .50 caliber barrel of 1:48 twist:

150 x .50 x .50 divided by 48 = .78 inches

The barrel will stabilize a bullet .78 inches long, or shorter.

©1997 B. E. Spencer
If you put in the .5 for a ball you get 75.

March 20, 2012, 07:56 AM	#1
Superhouse 15 Senior Member Join Date: September 13, 2007 Location: Sunny Florida Posts: 844	Round Ball? Minnie? Bullet? Twist rate? Help. Ok, I got my second black powder firearm, this one a .50cal inline percussion. At first it wasn't reliable firing the cap, but a little file work on the nipple fixed that. In the junk collected in the powder cabinet at the gunshop I found Minnie bullets and balls that fit, as well as some solid bullets that resemble the Remington "Buckhammer" slugs, but with wide grease grooves. The local D ick's sporting goods has sabot bullets in various weights. During test firing I loaded FFFG powder at 40gr behind a Minnie lubed with "Bore Butter" and it stabilized at about 20 yards and hit a paper plate. For convenience I made some paper cartridges (that worked... until I bit the end off like the guy on Youtube did, only I got some tasty black powder in my mouth). Development of those will continue. Anyway, now that it's reliable, does anybody know what style of projectile is correct with a given twist rate? Is there a chart somewhere or a rule of thumb? Anybody have any advice on powder weight? I do know two things, one: Minnie bullets need a certain minimum powder charge to expand and grip the rifling, and two: I don't want to blow my face up. Is there a projectile that works well in any barrel? It has an old but well kept Weaver scope on it, 4X, and I want to be able to shoot it at at least 100yds to take advantage of that. For hunting it's probably realistically going to be shot at 30 to 40yds max. And while I'm overloading everyone with questions, What about the pre-formed powder pellets? Some even come in a kit with sabot bullets and pellets of powder. Does anybody know how those taste? Thank you kind black powder Gurus.

March 20, 2012, 08:23 AM	#2
mikthestick Senior Member Join Date: January 29, 2012 Location: Devon England Posts: 177	The formula you want for twist is Greenhills formula I can give you it in a later post if you want it. For the moment so as not to confuse you. Lyman BP hand book say a 22" 1 in 24" twist barreled 50 cal Buffalo HP conical of 350grs @1148ft/sec with 60grs of 2f Geox behind it. The twist on barrels designed for balls is 1 in 48. The largest bullet on the chart is 490gr anything less than that should stabalise lovely in a 1 in 24" barrel. 120gr Geox gives 1551 ft/sec. Tell me your powder bullet/ball weight and barrel length and I'll print more info. A round ball requires less twist to stabalise than a conical bullet If your barrel is 1in 48 twist it will stabalise round balls a conical bullet is required to make the bullet heavier if you made one 200grs(not very conical) it would be much less stable than the round ball. It would almost certainly not stabalise. There is no chance a 350gr conical will stabalise in a 1 in 48" twist barrel I love maths and formulas but hesitate to print them as not everyone feels as I do. However if you want the info I'll help as much as I can. mik Last edited by mikthestick; March 20, 2012 at 08:59 AM.

March 20, 2012, 09:26 AM	#5
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,389	Most period round ball guns I've examined have twist rates of 1 in 50 or slower. One I saw years ago had a twist of 1 in 90. Thompson Center for years adopted a compromise twist rate in its guns of about 1 in 48. They would adequately stabilize both round balls and conicals. When Pennsylvania mandated round ball only, they came out with the Pennsylvania Hunter, which was a .50 with a twist rate of 1 in 66. The general rule of thumb is the shorter the bullet, the slower the twist rate needed to stabilize it. The longer the bullet (and heavier, generally) the faster the twist rate. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

March 20, 2012, 10:27 AM	#8
Beagle333 Senior Member Join Date: January 7, 2012 Location: Auburn, AL. Posts: 2,332	How to check the twist rate (Works the same on ML's) http://www.gunnersden.com/index.htm....ist-rates.html

March 20, 2012, 10:39 AM	#9
Hawg Senior Member Join Date: September 8, 2007 Location: Mississippi Posts: 16,189	Going by Beagles link. With a slow twist you're not going to have enough barrel length to make a full turn. I like to push mine all the way down to start with but it doesn't matter. Just mark it at the muzzle in line with the front sight. Push it in or pull it out whichever way you prefer until you get a half turn. Mark it again. Measure between the marks and multiply X2. Or you can go a 1/4 turn and multiply X4 I can't believe I said divide originally. Multiply X2 not divide as I originally stated. Last edited by Hawg; March 20, 2012 at 11:49 AM.

March 20, 2012, 09:05 AM	#4
mikthestick Senior Member Join Date: January 29, 2012 Location: Devon England Posts: 177	Please read my post again I have edited out my mistakes. My knowledge of ballistics is far superior to my command of the English language.

March 20, 2012, 09:37 AM	#6
Hawg Senior Member Join Date: September 8, 2007 Location: Mississippi Posts: 16,189	What Mike said. A 1:48 is a good medium ground for round balls and conicals but not ideal for either.

March 20, 2012, 10:11 AM	#7
mikthestick Senior Member Join Date: January 29, 2012 Location: Devon England Posts: 177	The rifle twists I mentioned are from Lyman BP reloading manual. If you buy a rifle how can you check its twist rate. I would have to take it to a gun smith.

March 20, 2012, 10:52 AM	#10
ScottRiqui Senior Member Join Date: January 27, 2010 Location: Norfolk, VA Posts: 2,905	Keep in mind that Greenhill's Twist Rate Formula isn't applicable to round projectiles, and it's really not ideal for any projectile that isn't at least 2.5 or 3 times longer than it is wide. You're better off just going with general guidelines and TLAR (That Looks About Right) than you are trying to justify a particular twist rate by mis-using Greenhill's formula.

March 20, 2012, 11:09 AM	#11
Superhouse 15 Senior Member Join Date: September 13, 2007 Location: Sunny Florida Posts: 844	Wow, thanks a lot. I am a gunsmith and as soon as I get to the shop I'll measure the twist rate. I loaded 40gr of FFFG in a brass measure behind the round ball and the Minnie. I also tried a load of birdshot strictly for testing purposes with 45gr but it was dramatically unsuccessful, not surprising considering the rifled barrel and small charge of shot. Bring on the math, please. My first black powder gun was a Spanish Deringer that I was finally able to safely fire with a patched ball and 25gr of the same powder. Still not exactly sure of an appropriate charge weight but I was cautious.

March 20, 2012, 08:04 PM	#12
B.L.E. Senior Member Join Date: December 20, 2008 Location: Somewhere on the Southern shore of Lake Travis, TX Posts: 2,603	Forty grains is a mighty light load in a .50 rifle. Accuracy suffers if a load is too light, not just for Minie balls but also with patched roundballs. Most people put around 70 to 90 grains in a .50 when shooting patched round balls, even heavier sometimes with the slow twist barrels.

March 20, 2012, 09:06 PM	#13
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,389	My Thompson-Center Renegade .50 flintlock wouldn't shoot well at all with anything less than 60 grains of FFFg. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

March 21, 2012, 12:08 AM	#14
Hawg Senior Member Join Date: September 8, 2007 Location: Mississippi Posts: 16,189	I used 90 grains in my .50. Oddly enough I use 90 in my .54 also.

March 21, 2012, 04:18 AM	#15
deerslayer303 Senior Member Join Date: January 10, 2011 Location: Leesville SC Posts: 2,653	40 grains???? Did the ball even make it to the target LOL . I'm just playing, but yea you need to put some FIRE to that rifle. 90 grains seems like a sweet spot in my .54. __________________ "I prefer dangerous freedom over peaceful slavery." - Thomas Jefferson

March 21, 2012, 06:57 AM	#16
Superhouse 15 Senior Member Join Date: September 13, 2007 Location: Sunny Florida Posts: 844	I was certainly taking a cautious approach to the first few shots. And I did mis type. The powder is FFFFG (four F's). I'll give it a boost and try starting at 70 and working up from there. I'm stuck at work today, going to measure and test fire tomorrow. I believe I'll wait on the Sabot bullets until I see what I can get with the Minnié accuracy wise.

March 21, 2012, 07:17 AM	#17
deerslayer303 Senior Member Join Date: January 10, 2011 Location: Leesville SC Posts: 2,653	I don't have no where near the experience as some of these guys here. But FFFFg is generally a pan powder for priming flintlocks. Fg is cannon powder, FFg a rifle powder, and FFFg a pistol powder. __________________ "I prefer dangerous freedom over peaceful slavery." - Thomas Jefferson

March 21, 2012, 07:59 AM	#18
Hawg Senior Member Join Date: September 8, 2007 Location: Mississippi Posts: 16,189	Don't use 4F for a main charge. Use 2F or 3F.

March 21, 2012, 09:21 AM	#19
Strafer Gott Senior Member Join Date: May 12, 2011 Location: New Mexico Posts: 1,315	Safety! I'd stick with the light charges until I knew what gun I had. Some of the legacy inlines are unsafe with major loads. A muzzle loader can be accurate loaded to different levels. If you are shooting for smiles load it light. I would not trust heavy loads without at least remote firing with a proof load. It's only prudent. Try googlin Randy Wakeman- CVA. A cautionary tale- at least something to think about.

March 21, 2012, 10:42 AM	#21
Fairshake Member Join Date: August 2, 2009 Location: Livingston, La. about 20 miles East of Baton Rouge, Louisiana Posts: 30	Powder and twist for BP I've been shooting BP since 1970 when I was working in a gun store. I purchased two C&B revolvers, one a 36 caliber and the other a 44 cal. I also purchased a 50 Cal Hawken rifle. The twist in a Hawken is 1-48 which is a middle of the road compromise twist. The best for round ball and patch is in the 1-66 range and slower. The new inline guns are not meant to fire round ball or the old conical bullets. They shoot the sabot rounds or the big conicals because they have a fast twist. The best for the conical or the new Hornady Plains bullets is the 1-48 and faster like the 1-36 or so. 2F is the best all around grain size for 50 caliber and above. If using Swiss then the 1 1/2 gets the call. All black powder is the exact same as far as it's production goes. It's a mixture of three chemicals and the speed at which it burns is determined by the grain size only. It all burns from the outside in and therefore the smaller the grain the faster the burn. The foreign made powders tend to be smaller than the Goex made in the USA. I have made several slides and viewed it under a microscope to verify this. Use your inline with larger charges and the correct componets and you will start to see good groups. Stay away from 4F unless you are primming a flintlock pan. 3F will burn in your inline but will more than likely not give the better accuracy. Each gun will have it's own favorite load. Inlines are usually made to fire 209 shotgun primers because of their nature of being harder to ignite the pellet type charges used by many people today. I have never fired a sub powder and never will as I'm a BP purist. You should buy one of the books by Sam Fadala which is in it's fith printing. It is named the Complete Blackpowder Handbook. I disagree with some of his writings but overall it's a good starter book to have. Later David __________________ SASS Frontier Cartridge class, 100% BP Shooter,

March 22, 2012, 06:24 AM	#23
Mike Irwin Staff Join Date: April 13, 2000 Location: Northern Virginia Posts: 41,389	I loaded 135 grains behind a lead round ball in my .50 once. Once. The stock didn't fit me all that well and it kicked the absolute crap out of me. It's a pussycat with 60 to 80 grains, though. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

March 22, 2012, 07:08 AM	#24
Nathan Senior Member Join Date: July 1, 2001 Posts: 6,331	I think it has been hinted at, but I'll say it. 70gr of 4F will likely blow you up. 80 of 2F would be a good place to start behind a sabot or conical of your choice. An average BP rifle is more than capable taking a deer at 125 yds.

March 22, 2012, 08:36 AM	#25
mikthestick Senior Member Join Date: January 29, 2012 Location: Devon England Posts: 177	Bring on the math Sorry I don,t like to write rubbish. My first post was a quote from lyman. What I wrote was BASED ON THE FACT THAT A 1 IN 48 TWIST STABALISES A ROUND BALL OF 50CAL. But that was an ASSUMTION not a fact, don't assume in maths or you risk embarasment below is the simplest explanation I could fin for Greenhills formula. 150 x diameter squared divided by bullet length = required spin Example for a .45 caliber bullet .60 inches long: 150 x .45 x .45 divided by .60 = 50.6 inches So, for the example bullet, a spin rate of 1:50.6 or faster is required The formula can also provide us with the maximum bullet length which can be stabilized by a given barrel twist. The formula becomes: 150 x diameter squared divided by twist rate Example for a .50 caliber barrel of 1:48 twist: 150 x .50 x .50 divided by 48 = .78 inches The barrel will stabilize a bullet .78 inches long, or shorter. ©1997 B. E. Spencer If you put in the .5 for a ball you get 75.