Bullet Pressure

[SGDM]

Does the pressure increase or decrease when the bullet length is increased. I am loading .40 cal and playing around with different OAL. I went from a length of 1.160 to 1.180 and found a drop in velocity thinking there was a reduction in pressure.

[Guest Dick W Holliday]

Funny you should ask because i'm getting my stuff together now to go the range to test a round that i just shortened pretty drastically.....i'm shooting a 40 that screwd me over the weekend with my long rounds of 1.175......i'm shortening them to meet factory specs.....i didn't reduce my load any so i'm going to chrono them to see what the velocity will be with the increased pressure of a shorter round....so to answer your question.....a shorter round with the same load will generate more pressure than a longer round with the same powder charge....D I C K

[Skywalker]

When playing around with .40 SW and Nitrokemia Rex-3, I found out the following:

my pet training load, 4.8 grs of Rex-3 over a 202 grs lead FP-BB bullet, 1.200" O.A.L. chronoed @ 870 fps, for a P.F. of 175 in my SVI Competition.

I had to reload the same load for a couple of fellow Glock shooters, thus I had to shorten the cartride to 1.120" O.A.L. to be able to stuff the rounds into the Glock mags (they do not allow longer than spec cartridges).

Well, to get the same velocity and P.F. I had to download the powder charge about 0.2 grains, settling down on 4.6 grs of the same powder.

The general consensus is that with the same amount of powder, the longer the cartridge, the lower the pressure, the lower the velocity you get.

[TBF]

I concurr.

Hope I spelled that right....

Travis F.

[Flexmoney]

A longer OAL should give less pressure...AND less velocity. But, know that pressure and velocity don't move in a one-to-one ratio.

Since 40 is a high pressure round...longer is generally considered to be safer.

Factory 40S&W OAL is 1.125 according to my data.

The Glock mags will feed out to around 1.165 OAL. (that is pushing it, I think)

I don't load my Glocks any shorter than 1.135 OAL.

Fast powders and short OAL's in 40 can be dangerous. Some powders are reported to have BIG pressure changes with just small changes in OAL.

[SGDM]

Thanks fellas,

I found a decrease in velocity using same powder weight, 3.9 grns Vit N320, 200 grn bullet, when I increased the length. I just wanted to confirm my data with ya'll.

God Bless America!

[Loves2Shoot]

3.9 VV with a 200 grn bullet loaded to 1.190 is my magic load for my limited blaster.

[Carlos]

LovestoShoot: You are using lead? Your load is close to what I am working up: 200 grn Westcoast Plated RFP at 1.155" OAL over 4.0 grns of VV N-320. I had to load at that OAL for use in a Steyr M40. I knew that plated and jacketed bullets require MORE powder to reach a given velocity than lead. I could not find a load for N-320 & plated 200s on Jeff Maass' website, so I used the data for lead 200s at a shorter OAL than my load. My theory is:

if its safe w/ lead at that weight of powder and:

its safe at a shorter OAL & w/ that much powder, then

my longer, plated load must be safe.

Functioned OK last night at the range; no idea what PF. Unfortunately it was "Cops shoot for free" night at the range and everybody showed up (no chance to set up chrono).

[zorba]

This may or may not help.

Recently i desided to try and lower my factor by changing oal only.

Results were good, i found i could taylor my factor to almost any thing i want.

EG. my original load in 40 was a 207.5 g lead bullet , oal of 29.4mm, factor 175.

Only chainging the oal to 29.9 gave me afactor of 169.

I then seated the bullet deeper to an oal of 29.7 and got the factor i wanted 171.

There were no signs of exesive presure , blaster is a para 16 40 and powder mp200,magnum sp primers.

[Skywalker]

Zorba,

yes you can achieve finer tuning of your pet load P.F. by playing with O.A.L. and even with crimp amount (more crimp => more pressure => more velocity) than with powder weight variations.

Anyway, usually, varying O.A.L. is something shooters do not want to mess with, because once you have found an O.A.L. that will give you 110% feeding reliability, varying that "magic" setup could cause feed failures.

In your case, you are talking of an overall variation of .5mm on a 30 mm cartridge: this is a 1.67% lenght variation, and it is unlikely such a small reduction will cause feeding failures.

But, as you have already pointed out, always watch for pressure signs when playing with O.A.L. and/or crimp amount, even if you are going on the "safe" side of the road.