Semi Auto Pressure Signs

[steve4102]

steve....tell us more about the contradicting data you have found. I am curious. I only reload for four common caliber and there is extensive load data on each (9mm, .45acp, .223, .243). I have never come across conflicting load data where a max load in one manual was a start load in another. Are you sure you are comparing the correct bullets and powder combinations??? Thanks!

Just off the top of my head I can come up with IMR 4227 in the 7.62x39. Hodgdon list 18.5gr as Max for a 125gr .311 bullet. Sierra # 5 list Start @ 20.6 and Max at 23gr.
http://data.hodgdon.com/cartridge_load.asp

You mentioned the 243. Lets look at that. Bullet Hornady 58gr V-Max, Powder H-4895.
Hodgdon: Start 40.0gr Max 43.0gr
Hornady #7 Start 35.9 Max 40.7gr.

243, 75gr Speer HP. Powder H-380
Lyman 48 Start 36gr Max 40.0gr
Speer #13 Start 39.5 Max 43.5gr
Hodgdon Start 40 Max 44.5gr


I could go on and on, but I think you get the point.

[IllinoisCoyoteHunter]

I do stand corrected. I think that the discrepencies in start and max loads boils down to how much these manufacturers want to cover their @$$. Now for a little nitpicking. Hodgdon does not have load data for a 75 grain speer HP...it has data hor Hornady 75 grain hp. Now are these bullets similar in all respects (bearing surface, etc)??? Enough to warrant the load discrepencies???? I honestly don't know....and don't really care LOL! Good luck and be safe!

[Stone Cold]

..

[Unclenick]

I took awhile composing this, so I apologize for duplicating information posted while I was at it.

I would suggest the OP run the search for threads on the topic of pressure signs. There are many and none are wholly reliable, so you watch for the first one(s) to show up and decide whether or not it is safe to try to keep going? There is experience and judgment involved as some signs appear way too soon sometimes and don't show up until it is too late at others. In the specific case of a sticky bolt (one sign) the closest corresponding behavior in a gas gun is usually the extractor pulling too hard on the rim off the case because the case isn't coming out as easily as usual. Look for an extra-deep extractor mark in the case rim or a bent case rim. Some brands of brass are softer than others, so don't be surprised if the same load that bends the rim of a Hornady/Frontier case leaves a Lake City case looking fine.

All the common pressure signs below are unreliable in isolation, but all throw up a caution flag. Accumulating pressure signs is like accumulating evidence. Most modern guns withstand more pressure than SAAMI max, as the proof loads have to demonstrate at the factory, but I can't see beating the metal up for no reason. Be conservative. But if you detect, say, casehead expansion, yet your primers are very rounded at the corners, figure the CHE was premature (it can happen bellow 40,000 psi one time, and not until 70,000 psi the next). It's a question of looking at the whole picture.

• Primers really flat
• Primers crater (raised rim around firing pin strike; but also caused by oversize fining pin tunnel or undersize firing pin).
• Primers mushrooming (fired primer head wider than sides of primer pocket, forming a slight mushroom head)
• Primer piercing at firing pin strike (also caused by too much firing pin protrusion or striking force)
• Primers leaking around primer pocket (also caused by under pressure loads, though usually accompanied by high primer if low pressure is to blame)
• Primers fall out when you try to re-prime the case (case head grossly expanded)
• Casehead expands more than half a thousandth of an inch on firing*
• Case pressure ring (the shiny area just ahead of the case) expands more than two thousandths*
• Case primer pockets get loose in 5 reloadings or fewer
• Cases are hard to extract or bolt is sticky
• Case rims look like about to be torn off due to sticky case on extraction (also caused in gas gun by using too slow a powder raising gas port pressure)
• Cases swell into extractor groove (primers usually are loose and/or fall out)
• Cases can't be re-chambered after extraction (does not usually apply to gas guns owing to case stretching on rapid extraction; also can be caused by off-axis chamber if case is not reinserted with same orientation)
• Groups on target start to open up with charge increase
• Velocity fails to increase when charge is increased
• The gun sounds or feels different or the shock you feel in the stock gets worse
• And the obvious: Blown cases, smoke appearing where it doesn't belong, broken metal. You went way too far if you get here.

*see: http://www.shootingsoftware.com/ftp/...%2019%2004.pdf

I'm sure this list is not complete, but those are the common ones that come immediately to mind.


Pressure verses velocity.

Unless the barrel is very long relative to the powder charge size, the bullet is accelerated to some degree all the time it is in the barrel. Most during the pressure peak, least as it approaches the muzzle. That acceleration is determined by the average pressure applied to the base of the bullet during its time in the barrel, minus the friction that opposes that pressure. A pressure gradient develops between the bullet and the breech end. It is small when the bullet is near the chamber and not moving fast, but the pressure drop from the chamber to the bullet base grows as the bullet speeds up and the gas has to chase it to keep pushing.

So, pressure measured at the chamber always exceeds what is actually pushing on the bullet. Peak chamber pressure is not what determines final bullet velocity. The average pressure at the bullet base minus the friction is. That average is the result of integrating the curve of the projectile base pressure minus friction to determine the area under the curve, which, as mentioned earlier proportional to actual acceleration.

where: 
a = acceleration
P = pressure
Fb = friction
t = barrel time
m = bullet mass
A = bore cross-sectional area

      t
a = m ʃ (P/A)-Fb dt
      0

Here is an example of the same pressure in the same gun giving different velocities with the same powder:

.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with Varget in gun with SAAMI minimum chamber produces:

18" barrel MV = 2575 fps

24" barrel MV = 2755 fps


.308 Winchester, Winchester brand case with 59.5 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with Varget in gun with SAAMI minimum chamber produces:

18" barrel MV = 2610 fps

24" barrel MV = 2797 fps


Here is an example of the same pressure in the same gun giving different velocities with the different powders:

.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with IMR4198 in gun with SAAMI minimum chamber produces:

18" barrel MV = 2555 fps

24" barrel MV = 2712 fps


.308 Winchester, IMI brand case with 56 grains water capacity, 150 grain Hornady FMJ boattail, loaded to 50,000 psi with 748 in gun with SAAMI minimum chamber produces:

18" barrel MV = 2668 fps

24" barrel MV = 2856 fps


Note that you have to have the same barrel length as the factory or manual test load was fired with for listed velocities to come close to applying. Moreover, your bore cross-sectional area, which depends on groove diameter, bore diameter, and rifling configuration all affect the amount of bore friction as does the roughness of your bore. That friction is subtracted from pressure at the bullet base, as shown above, so it affects bullet acceleration. If any of these factors in your gun is different from those in the test gun used to develop the load, pressure and velocity achieved will be different in your gun. Add to that, the internal volume the case expands to in your chamber determines peak pressure, and with it, powder burning speed. That probably isn't identical in your gun either. I have fired the same lot of M72 match ammunition in different M1 Garands and found average muzzle velocities that varied from 2495 fps to 2610 fps. And that was with a statistically insignificant group of six guns. If' I'd had more samples to test, I'm sure that would have opened up.

Warning!:
Taken together, and ignoring the fact you likely have no way to independently determine the accuracy of your chronograph, the last paragraph illustrates why you must never rely on a chronograph to determine a load level. Your gun won't get the exact same velocities for the exact same peak pressure that the test guns did. Every set of chronograph instructions I have ever seen warns what an unsafe practice that is. The only time a chronograph is useful in indicating load limits is when its average readings, whatever they may be, fail to increase when your increase your powder charge. That is a pressure sign.

Below are some QuickLOAD plots to satisfy curiosity. The first is chamber pressure vs. distance the bullet base has traveled (red trace) and bullet velocity gained vs. distance down the tube (blue line). The second is bullet base pressure for the same load vs. distance the bullet base has traveled.





Nick

[two 70]

This post is an excellent read, probably should be a requirement for new handloaders but the fact still remains that loading info varies to an extreme that can become confusing. Case in point for me as I just got back into reloading is searching for that magic 3140fps .270 load with a 130gr bullet. Seems I remember using 4350 back in the 60's but can't remember the load. Probably around 55 grains. My Hodgdon booklet recommends a 4350 max load clocking way under 3140 so I checked my ole standby books by Jack O'Conner who swore by 4831. Manual shows a compressed max load of 60 grains @3000 plus but according to ole Jack, he was achieving remarkable velocities of over 3200 with 62 grains of 4831. Must have been using damn site longer cases than I do cause I can't seem to get that much 4831 in a standard OAL .270 case without a hydraulic ram. Pressures must have been impressive too as the 60 gr max load indicates 51,000cup according to Hodgdon. Guess ya'll can just put me in with the nit pickers but inquiring minds do want to know why.

[Kjeil]

I wish I would have read this thread BEFORE I went to the range today to shoot by first set of loads. It turns out I was looking for the wrong thing, I didn't know about watching the primers, I was just watching the base of the cases and the necks. Here's the details, please let me know what you think.

I had 17 batches of 10 rounds with various combinations of 55gr, 69gr, and 75gr 223 using Varget, N135, and 3031. Most of them went fine but something didn't seem right with one of the batches.

I noticed more muzzle flash than normal and one time I fired I thought I saw a flash out the side. When I checked the target I saw this:



The range was only about 50 yards but that was the first time I had ever saw anything like that.

After reading this thread I checked my primers and they do look pretty flat, here's a picture. Is this what you mean by flat primers?



The one on the far left is a factory Remington 55gr and the one on the far right is a unloaded, unfired case that was primed as the same time as the others.

The load was:
Hornady 75gr BTHP
N-135
21.3 gr
CCI #41 Primers
WCC once fired brass

It was the one just below the max load in the Hornady manual (the conservative? one), it should have produced 2500 fps.

The rifle was a
CMMG AR chambered for 5.56
Chrome lined 16in barrel 1:7 twist
Mid length gas system
It's only a couple months old with about 300 rounds through it so far

I also noticed that the brass ejected much faster and at a different angle for the batches with the 3031 and 75gr (loads were 19.2 and 20.5). Instead of bouncing of the wall beside me the all went right over my shoulder. But the felt recoil wasn't bad at all, in fact it seemed less than the 69gr Sierra. The grouping was among the best.

So one question I had is that when your comparing to factory loads (as mentioned above) you should probably compare them to similar loads right? Would you expect some difference between 75gr and 55gr (in terms of ejection and recoil) or should they be about the same?

Unclenick, thanks for posting the math. I've been wondering where to find something like that so I can put all that calculus I took to good use.

[steve4102]

Ejection in an auto-loader is caused by "port" pressure and not "chamber" pressure. What this means is that the gasses generated by the burning powder travel down the barrel and enter the gas port. The gas port is a hole in the barrel that accepts these gasses and forces the action back and ejects the brass.

In general slower powders produce more gases and thus will toss the brass further and cycle the action harder. That is NOT to say that these powders are producing more "chamber" pressure, just more gas going down the bore and entering the gas port. On the other hand, fast powders produce very little gas, some so little that the action will not cycle at all, even though the chamber pressure is very high or even too high. I hope I explained that well enough.

As for your pictures and your primers, everything looks OK to me. I checked your load with all the sources I have available and I think you are just fine. If it is an accurate load I see no reason to change it.

[Shoney]

Pressure Signs In Semi-autos: Unclenick said it very well.

The contents of this thread should be a testimonial as to Why You Need Several Manuals and Online Data Sources.

What new loader haven’t been coached on is YOU MUST FIRST Look AT Information OTHER THAN MIN-MAX LOADS, and match the loads closest to your situation;

Compare
1. Weapons used in the test
___a. Manufacturer
___b. Barrel length
___c. Action
2. Case manufacturer
3. Bullet details (see below)
4. Where the tests were conducted (although this is seldom available, altitude makes a difference)
5. Temperature - Some data also includes this

WHY?????
Because combinations of factors can cause great variances in velocity and pressure. Here are a few factors:

Primer: strength, brisance - is a measure of the rapidity with which an explosive develops its maximum pressure
Barrel: length; tightness of bore; height of the lands; distance of bullet to lands; temperature of barrel;
Bullet: bearing surface of bullet, alloy of bullet; shape of bullet; seating depth;
Brass: new/used elasticity; manufacturer, volume;
Powder: new, aged, old, batch powder was from;
Weather: ambient air temp., barometric pressure, humidity
Elevation: above sea level
Other: I am sure I have not listed all

Now, mix and match them, Care to guess the number of possible combinations?

“Bottom Line” Use the data a guide. Use all the available information to make your choices.

[C Bass]

What's the cause of the black swirl marks around the bullet holes in his paper target? Powder residue the copper jacket picked up in the barrel?

[Shoney]

Kjeil
The 55 gr bullet in your photo appears to be just starting to come apart. The 55 grainers are not intended to be shot at 1:7 twist at high speed. That high rate of spin is probably tearing them apart.

Did you see what happened at 100 yards? If what I am saying is correct, they probably won't get to 100 yards in one piece, if at all.

From the VihtaVouri reloading data, using a 25" barrel:
Bullet ___________ Twist _ OAL __ Powder _ Grs __ FPS___ Grs __FPS
75 BTHP Hornady -- 1:7 -- 2.260 -- N135 -- 20.7 - 2467---23.3 - 2723

[SL1]

When comparing bullet velocities listed in loading manuals, you need to remember that those velocities are usually NOT the velocities from the test barrels which produced the pressure readings. Usually, max loads are developed using the test barrels, and then those same loads are shot through a typical production gun to give realistic expectations of velocities you can get. That makes it much harder to compare data between sources, because pressures and velocities for IDENTICAL rounds can vary so much among typical consumer guns (compared to variations between different test barrels, which are made to MUCH tighter specifications). So, even if two pressure test labs used the same test barrel and measuring equipment to produce EXACTLY the same load vs pressure data and gave the same max charges, they would print different velocities in their manuals if they used two different commercially available guns to get the velocities (even two commercial guns with the same barrel length from the same manufacturer). When you throw in the fact that they do use different pressure barrels and measuring equipment, plus different lots of powder and primers, and usually different bullets, the relationship between max pressure and velocity (or max charge weight and velocity) becomes much less clear to the handloader.

So, as others have written, simply using a chronograph to match your own handload velocities to those in the max load data of a manual is not telling you much about the pressure your handloads are producing.

SL1

[Kjeil]

Shoney,

I'm not sure if I understand what you're trying to say.

The holes in the picture are from the 75gr BTHP, not the 55gr. If they were starting to come apart I don't think the holes would be that uniform would they?

I have been trying the 55gr as well but with appropriate loads from the manuals, usually the lower end. I've been trying to be pretty careful because of the twist issue. I started by shooting about 300 rounds of Winchester factory 5.56 55gr FMJ and that went fine. Then I did about 60 or 70 of 55gr vmax without any issues so far. Of course I haven't gone over 50 yards (because of the range I'm at) so maybe that's where I'll see issues.

But overall the 75gr performed better and the cost isn't that much higher when I load (which is why I started loading in the first place) so I'll probably stick with those.

C Bass, I'm still not sure what those marks were from but I don't think it is residue. After that load I did another 30 or 40 rounds and didn't see any more marks.

[Sam06]

Very good post and explanation Unclenick! I think Shoney is correct on the signs also. Here is what i do when confronted with different max loads for the same powder and bullet combo. I average them and subtract 1.5gr-2gr. This will "Normaly" Put you in the upper 1/3 of the hottest loading data. Also get the newest loading data you can get. If you are loading using a lyman 1955 load book for say 2400 and loading to the max, you will be in trouble fast. Most powders have been reformulated or are made by a different company now than even a few years ago.

The other thing is in 308(which I believe you are loading for) fast is not always good. If you are shooting for accuracy and using a 168gr SMK or 175gr SMK or similar bullet keep the veloicity in the 2600-2675fps range and you will see good accuracy. For 155gr Palma they like to run in 2800+ range but no more than say 2880. It will depend on the twist of your barrel.

When loading for the 308 and looking for accuracy check out this site. They have good data and they ONLY seek accuracy..............Sam

http://www.6mmbr.com/308Win.html

Edit sorry for the mix up, you are shooting a 223. I just looked at the photo of your bullets and they do look like the primer was flattened. The powders you listed are all good ones. Here is the sight for the 223:

http://www.6mmbr.com/223Rem.html

You are using Military brass so you should reduce your loads by 5-10% because of the thicker brass with less volume. The bullet holes look as if they are coming apart. 75gr bullets should be ok in a 1:7 twist barrel. You are a least a grain over and you are pushing the bullet too fast for that twist. Slow down and the above is true for 223. Push the bullet to the upper mid range in speed and it will be very accurate. N135 is a good powder but it is snappy so be careful.

[C Bass]

You are using Military brass so you should reduce your loads by 5-10% because of the thicker brass with less volume. The bullet holes look as if they are coming apart

According to the link you posted, LC brass has some of the highest internal capacity so it would appear that it's not necessary to reduce loads. Is that correct? This surprises me because I've always heard that it is thicker and would produce more pressure than commercial brass with the same load.