Shorter OAL but No Powder Compression

[chiltech500]

I've switched bullet types 3 times on my 9mm cartridges from RN lead to SWC lead to TC lead and each time the usable OAL decreased because of the lube strip location. Plunk test says my longest OAL is fine - OAL's are: 1.155 for the RN; 1.12 for the SWC and now 1.075 for the TC.

I've been using Bullseye 4.0 gr for all. The powder is not being compressed by any of the bullets. Assume bullet weights are the same, is there much of an increase related purely to OAL?

Thanks in advance.

 

[chiltech500]

Effects of Shorter OAL on Pressure - Without Powder Compression

Wow no responses, very unusual, maybe I should PM a couple of fellas who have had a lot of answers for questions in the past.

 

[rcmodel]

No mention of the bullet weight you are using.

My crystal ball is in the shop betting a buff & wax job.

If you are using 115 grain bullets, your 4.0 grain load is well under Max.

Nothing to worry about.

If you are using 147 grain bullets, there would be.

rc

 

[32_d3gr33s]

Are all the bullets the same length? Oal will not affect pressure if, say, you have a shorter oal with a swc than a round nose, but the bullet length is also shorter on the swc than the round nose. So lengths of all 3 bullets would also help...

 

[chiltech500]

Sorry rcmodel, I was looking for a generic answer to a generic question; I probably messed up the question with too many details. Let me rephrase the question:

Given all else being equal, what is the effect of a shorter OAL, with the same weight of powder but no powder compression, on pressure and fps?

In real life Bullseye 4.0 gr on 125 gr LSWC in 9mm is near the top of the weight suggested by various knowledgeable folks; Alliant does not have details for this specific bullet and weight.

 

[gamestalker]

The generic answer is, OAL does have a varying impact on pressures. With higher pressure cartridges such as 9mm and .40 cal. the increase or decrease in pressure can be some what significant if it involves .010" or more OAL variance.

The rule of thumb with handgun cartridges is to always work your charge up at the shortest OAL first, then you can increase the OAL with less concern of a pressure spike.

GS

 

[rfwobbly]

The powder is not being compressed by any of the [3 types of] bullets. Assume bullet weights are the same, is there much of an increase related purely to OAL?

While what Gamestalker said is true of one bullet, however it may not be true when discussing 3 vastly different bullets, as you are.

The problem with your question is that it assumes that each of the 3 lead bullets are the same length, same lead hardness, same barrel fit, etc, etc. In practice though, 3 different bullets will hardly ever share many of the same physical qualities. Therefore, their performance specs may vary wildly..... or they may not. The only way to know is to chrono the loads side-by-side and see.

So 'no', because there are so many variables in the mix, there is no general rule in effect that will allow you to make the generalized assumptions you're looking for. That's only one reason every reloading manual advises you to begin at the "starting load" and work up.

 

[gamestalker]

Rfwobbkly is also absolutely correct. Any time you change bullets, and in this case we are talking about 3 different projectiles, a complete new work is the standard approach.

What I was referring to is, OAL as a general rule, can have a significant impact on pressures. How much impact, well that's what one must ascertain through the work up/evaluation process, and shouldn't be an assumed result.

GS

 

[dickttx]

I don't believe it is compressing the powder that causes in increase in pressure. It is the reduction if space inside the cartridge. If the oal of your bullets are not the same then you can't relate to the OAL of the cartridge.

 

[chiltech500]

Thanks gents, I guess I need to measure the amount of bullet below the lube strip and compare those. Somewhere I read that compressing the powder creates a big rise in pressure. It makes sense that having a smaller chamber but not compressing also has an affect.

I reload for Bullseye so I do not seek heavy loads and am not pushing any envelopes. That said, I initially purchased Round Nose and used data for them and then purchased the SWC and used data for them. I intend to shoot SWC all the time now.

I recently bought the Truncated Cone (where no exact data was available) to compare them to the SWC and immediately decided that was the last time I would use them. The TC bullets were 3 grains lighter than the SWC so I felt ok using the low end of the powder I used for the SWC.