velocity difference with regards to case capacity

[davewilson]

if you have 2 cases that hold 1 grain difference in total amount of powder, but you put the same amount of powder in each one. all other things being equal,will it make any difference in the velocity? i understand this is why we separate the cases, but has anyone actually tested this to see if it makes a difference?

 

[royinidaho]

Dave,

On the fine end of the spectrum Goodgrouper would say you betcha, definitely..

At the other end of the spectrum there's folks like myself who just don't seem to care that much, which requires, in addition to weighing brass, using a nearly lab grade chrono and matching primer weights.

My experience has been with using a single case and seating the bullet longer and checking velocity and have found that velocity is inversly proportional to case volume for the most part, well almost all of the time, pretty much without sorting primers.

 

[Mysticplayer]

Interesting

From my experiences of measuring some brass, I would be really surprised if you could find such a situation using brass from the same brand and lot. You might find this in military brass...maybe.

Most brass I have measured varied very little, if any at all, as far as case volume went AFTER fireforming.. This has been very interesting to me considering that I have been working with 'crappy' domestic brass.

For me, I want the combustion volumes of the cases to be as close as possible. They don't have to weigh the same.

You will find this level of difference by using brass from different brands. There will be a velocity difference but who is to say that wasn't also affected by such things as annealing, brass alloy/elasticity, flash hole dimensions, etc, etc

As to an exact amount, just grab different brands and find two with the desired volume difference and fire them. Post the results as it would be interesting.

Jerry

 

[steve smith]

One question!

What is the case capacity you're speaking of?

20gr case capacity might make a whole helluva lot more difference then 120gr case capacity!

Just a thought!

 

[royinidaho]

ya'll talkin' to me?

I sort my cases into +/- 1 grain. Out of 50 REM 300 RUM cases I end up usually a few more than 30+. I then fire form them to 270 AM which increases volume a bit more.

What I think I'm learning is that there is a bit more sensitivity to any of the variables at the extreme end of things such as a 277 195gr @ 3200 than say a more typical cartridge at more or less factory velocities. Don't know for sure. Not quite enough experience yet. There's a reason GG goes to the detail that he does and gets the results that he does....

Additionally, I think that the AM's accuracy wouldn't fall off noticeably out to around 400 yds plus if I didn't sort brass at all. But for decently long distances, 500 and beyond the vertical spread is noticeable and gives me the vapors.

 

[Mikecr]

Case volume variation by % definitely makes a difference with a given load..
You can play with this using QuickLoad. Or you can measure velocity difference between unfired and fired brass, for a rough sense of it.

Don't even bother weighing cases.
Measure H20 capacity.

 

[davewilson]

i'm with Mikecr on this one. i think weighing cases is a waste of time. i thought we're after the inside volume of the cases being as close as possible was the goal. case in point, i just measured 50 pieces by weighing the amount of powder each will hold. mine is a RUM case and 2 of them are right at 111 gr of powder and 2 others are right at 112 even. 3 of these pieces of brass weigh exactly 270 gr and the other is 272.5 gr. i double checked all the weights to be sure. the extreme difference between all of them is 2.6 gr. hence my original question is how much will 1 gr of powder capacity difference make if the same amount of powder is used.

 

[steve smith]

Dave

I would have to say that for the 300RUM which is what i consider to be a high capacity case, that one grain of powder capacity would be very hard to detect, as far as a difference in velocity.

I mean most people won't be getting velocity spreads small enough to detect any potential change in velocity.

With my 270-300RUM, it was usually tough to see any increase/decrease in the velocity until I bumped the powder charge by 2-2.5 grains or so, and sometimes there would be no change in velocity. The powders that are available just aren't that consistant.

Just my opinion!

 

[royinidaho]

Dave

The powders that are available just aren't that consistant.

Ain't that the truth!!!!!!

I posted this before, maybe here???

US 869 - 195gr RBBT - non-sorted cases-Fed215GM
Grains Kirby's MV My MV
88 2771
89 2933
90 2957 2903
91 2993 2945
92 3024 2968
93 3041 3009
94 3070 3036
95 3131 ????
96 3146 3150
97 3192 3201

Mine were shot in sub 20* weather.

Steve, I got the same results as you explained with using 135 and 150 gr bullets. The 150s topped out at 3400+ and shot great.

I've just started on a new batch of REM brass (+/- 1gr). Its a bit different from the first batch that I used up. Velocities are just a bit faster maybe 30FPS.

 

[davewilson]

did a little testing this morning using cases that held exactly 110-111-112 gr of powder and kept every thing else the same. including weighing the primers.numbers are the velocity of each shot other than the first one where i changed the battery and got a reading every time from then on.

110 111 112
xxxx 3091 3081
3083 3095 3120
3100 3111 3096
3112 3095 3114

3098 3098 3102 averages of each

i was a little surprised and disappointed by these results. of course it's only a few shots, strings of 10 would have been a little more accurate. the averages aren't as far apart as i thought they might be. might do another round using the same cases and see how they compare.as of right now i would have to say that even cases with 2 gr of powder capacity difference doesn't make much difference at all!