Do short barrels amplify velocity spread?

[Pond, James Pond]

As the title suggests.

I did some chrono work today. All the cartridges were loaded with Hercule Poirot-like attention to detail on my Lee scale. The beam was spot on every time!

They were all seated and crimped at the same time.

However, I got some pretty impressive spreads on my chrono readings at times. Once even 150fps difference!

The two things that are worth bearing in mind are that I had very small lots to shoot (3 per charge weight) and I was standing about 3m from the chrono.

So could the spread just be down to using a tiny snub?

[Beentown71]

I like being 15m or further. I get erratic numbers from muzzle blast with certain calibers.

[Pond, James Pond]

I like being 15m or further.

Where I test I don't have that much space. Besides, at that range I'd be seriously worried about putting one through the chrono so far away!!

I've already once put a huge kink in one of the disperser struts.

Aside from that, could barrel length be guilty?

[10 Spot Terminator]

The spread is more controlled by the powder burn rate and the bullets that are combined with the powder . What works in a snubbie generally doesnt perform as well in say a 6 inch barrel of the same calibre and vice versa . You didnt say what powder, bullet or even calibre you were shooting so this is some generic kinda sorta place to start info .

10 Spot

[Pond, James Pond]

In this case it is N320 with 125gr copper-plated, polymer coated bullets.

[WESHOOT2]

no

[Jim243]

What Weshoot2 said.

Try it again from 4 meters this time (further from the muzzle blast) and take a 22 LR with to test the crony before shooting the other bullets to see that you have it set up right.

Jim

[buck460XVR]

Yep...at 3 meters you may have been measuring muzzle blast and/or unburnt powder. How was the light?

[Pond, James Pond]

How was the light?

Mid morning, bright, but overcast.

If this means I have to redo my loads I will NOT be impressed!!

Having said that, they all fired well, and none showed signs of sticking on extraction, so I think I may be OK with what I have already tried.

Where I do my load testing, I may have to put the chrono almost up against the target!!

[bfoosh006]

What specific round were you reloading for ?

If .38 Special, and given a charge weight of about 5.0 grains... where your powder rests in the case , by the primer or towards the bullet , will make a big difference in FPS and spread. Next time if possible, shake the gun and powder towards the muzzle , fire 5 shots ( shake each time )... record info , then do the opposite... shake the gun and reloads powder towards the primer end...fire 5 , shaking each time, record info and compare FPS.

You will see the difference the powder position can make.

[steveno]

you don't say what caliber you are shooting. 10 feet in my opinion is more than enough distance for the first screen with just about any handgun caliber. I have got what I'm satisfied with magnum handguns at 7 to 10 feet with 41 and 357 mag calibers. it might not be enough for most rifle calibers in which case maybe 12 to 15 feet. I think you need to shoot 10 of a particular load to get a better idea of what a load is doing. I don't think barrel length will have much to do with spread

[44 AMP]

While I have no personal experience with the powder you used, I can tell you that some powders require a certain "load density" to perform at their uniform peak. And this can include the barrel length they burn in as a factor.

Powder position in the case when fired was mentioned, and it does play a part in the uniformity of the load. That is why it is generally considered to be a best practice to use a powder that not only gives the desired pressure and velocity, but ALSO fill the available powder space the most.

3m should be enough space to keep the chronograph from measuring muzzle blast, but might not be enough, add one more meter to the distance and try it again to see if there is any difference in your readings.

Also, I have to tell you that such a small sample size (3 shots each load?) doesn't tell you quite enough for a good read on what the load will do overall. IT does tell you some things, but only some things, and other things might show up with a larger sample.

[Bart B.]

The only way to get significant results from long versus short barrel is to use a long one first, then cut off a few inches and make the same face and crown on it, then shoot it. All with each test made with at least 20 rounds of the same lot of ammo and the barreled action fixed in a hard mount so a human's inconsistant hold of the rifle against their shoulder wouldn't corrupt data.

[Pond, James Pond]

Also, I have to tell you that such a small sample size (3 shots each load?) doesn't tell you quite enough for a good read on what the load will do overall. IT does tell you some things, but only some things, and other things might show up with a larger sample.

I absolutely agree but these triplet sets were just to see if there were signs of over-pressure and looking at the general velocity trend was part of that.

I had 18 different loads each with a single different parameter and making 10 cartridges of each would have probably taken all night!!

I have chosen some specific loads to try again and they will have bigger sample sizes (12-18 each).

[Jim243]

I had 18 different loads each

Ouch, I usually only do 4 different loads (5 each, 2 for function and 3 for accuracy) and that seems to work out. (generally only 3) Take the smallest group from the 3 and work from there.

Jim

[Pond, James Pond]

Ouch, ...

Indeed!

I had been feeling the drawbacks of loading for a snub, having had a couple of squibs, light loads, and unburnt powder clogging up the cylinder and internal faces etc.

So I wanted to try the same or similar charge weights with two lots of different OAL, a different crimp and mag primers. When you have between 4 and 5 charge weights being repeated with one of those 4 parameters, it soon adds up!!

[WESHOOT2]

List your available powders, bullets, primers, and GOAL.

We can help

[Pond, James Pond]

List your available powders, bullets, primers, and GOAL.

We can help

2" .38Spl
Powders: N350 (retired to .44Spl duty), N320 current choice
Bullets: H&N plated truncted cone 125gr
Primers: Mostly Fiocchi small pistol, some Remington small magnum pistol.

Goal: Not blow up gun with squib loads while trying to develop a plinking/practice round! (previous attempts gave mouse hiccup loads and unburnt powder aplenty)

[WESHOOT2]

Your N320 should be fine.
Your primer choices are fine.

Now load some test ammo using sorted headstamp cases. Same primer. If you can, slightly lengthen your OAL.
And for the next test samples, shorter.

And might I recommend perhaps a firmer crimp?

Any change?

[Pond, James Pond]

Your N320 should be fine.
Your primer choices are fine.

Now load some test ammo using sorted headstamp cases. Same primer. If you can, slightly lengthen your OAL.
And for the next test samples, shorter.

And might I recommend perhaps a firmer crimp?

I've sort of already done all that.

I took my normal OAL loads that had been a bit weak despite being on the strong side of certain load data (not for exactly the same bullets) and reloaded them once with an OAL .5mm shorter.

I loaded the same loads again with yet another .5mm taken off, so from 36.5mm down to 36mm, then 35.5.

Then I loaded another set of those same charges with the original 36.5mm OAL, but all with an extra quarter turn crimp so from a half turn up to 3/4 turns.

Finally, I loaded the same charges, but with an additional one to start a bit lower. These too had an OAL of 36.5, but these had mag primers.

So I had my original data from the first loadings a few months back, then two sets with incrementally shorter OALs, one set with just a tighter crimp and another with just mag primers...

None felt weak and I'd be happy to use some of the charges for the mag primers and tighter crimp. I had planned to load quite a few of each to then give the chrono a fair few to work with.

Please don't make me load up the whole lot again though!!! I beg thee!
Took me ages and I got a splitting headache from working hunched!

[44 AMP]

James, get a stool you can adjust the height. Too high, or too low is never a good thing.

[Slamfire]

This is a tricky question. I have collected a lot of chronograph data with Bullseye. Based on an examination of what I saw over the screens, there is not a significant difference in extreme spreads between a 2" barrel and 4" barrel. However, I am limited to 32 data points on a Chrony, and I have not gone looking through my database at other powders.

   Colt Detective Special mfgr 1963 
   	
158 gr LRN 3.5 grs Bullseye thrown, CCI primers mixed  cases		
24-Jul-99 T = 100 F !					
Average 751 fps		
Standard Deviation 32 fps
Extreme Spread 90 fps
High 805 
Low 715		
Recorded shots 22		
					
158 LRN 3.5 grs Bullseye WSP Mixed cases	 8-Jan-06 T = 52 F
Ave Vel =	675.9				
Std Dev =	18.6				
ES	96.18				
High	723.9				
Low	627.7				
N =	32				 
					
158 LSWC 3.5 grs Bullseye WSP Mixed cases	8-Jan-06  T = 58 F
Ave Vel =	706.8				
Std Dev =	15.55				
ES	65.87				
High	742.9				
Low	677				
N =	32


					
Colt Police Positive  Special 4" Barrel  mfgr 1930	
					
		 	
158 LRN  3.5grs Bullseye Mixed cases Fed 100 	4-Sep-05  T = 72 F
Ave Vel =	759.2				
Std Dev =	36				 
ES	104.8				 
High	813				 
Low	709				 
N =	32				 
						
158 LRN  3.5grs Bullseye Mixed cases WSP 	29-Jan-06  T = 68 F
Ave Vel =	760.3				
Std Dev =	27.53				 
ES	111.4				 
High	814.1				 
Low	702.6				 
N =	29				 
					
158 JHP  3.5grs Bullseye Mixed cases WSP  29-Jan-06 T = 68 F
Ave Vel =	633.1				
Std Dev =	41.32				 
ES	113.2				 
High	706.7				 
Low	593.5				 
N =	6	

[WESHOOT2]

First let's determine if it's ammo- or gun-related:

-using one specific load, chronograph said load using the same chamber.
Mark one chamber and use it exclusively.

Now how's the ES?

[Pond, James Pond]

I'm happy to try the "one chamber" thing, but I will first try the "take a few steps back from the chrono" thing!!

That seems the most likely explanation for me at this stage.

I also want to prepare as many "tests" as I can to record the results in a single range visit, if at all possible.

I have far less time to go to the range as I'd like so I try to kill as many birds with the one stone as I can.