9 major bullet weight vs gas

[lroy]

If i have 10gr of powder in two cartridges, one 115 and the other 124. Is the amount of gas working the comp the same?

 

(Let's pretend they both make major and you're able to offset the bullet size to make them identical oal.

[Alaskapopo]

I don’t think you would the difference either way 

[shred]

Same powder or different?  Where's the pressure peak?

 

[lroy]

8 minutes ago, shred said:

Same powder or different?  Where's the pressure peak?

 

 

Same powder. Suppose I'm asking if the powder charge is the sole thing affecting the comp.

Edited April 7, 2023 by lroy

[shred]

The powder gas is most of it but just the weight is not the only thing.  Burn rates and barrel pressure have an effect too.  Plus barrel volume as the air pushed ahead of the bullet does a little.

 

But it's largely irrelevant.  What are you trying to accomplish?  Recoil and flip and feel are much more than just what the comp does, although it can be lots of fun to noodle on comp designs.

 

 

 

 

[lroy]

Suppose I was just trying to figure out how things work. Maybe I've just been reading too much online.

 

For example, that test where you fire against a no shoot and see how much splatter comes out the front of the barrel. Theoretically, there is a maximum amount of gas the comp can handle and you want to achieve that point and no more, correct? So if you reach that point with a heavier bullet, is there no reason to try a lighter one, as the excess would only exit the front of the barrel and hit your hand?

[Farmer]

I would think the lighter bullet with the same charge would decrease your burn efficiency. Less weight, less bearing surface, less friction, faster acceleration that drops psi quicker ect. That may result in more splattering in your test. Don’t know until you try it. 

[Brazos Custom]

To compare you would want the case volume when the bullet is seated to be the same, not the same OAL. Shorter bullet would need to be seated further in. Pressure peaks in the first couple tenths of an inch. Small changes in the amount of case volume can mean big pressure spikes. That's why people worry about setback. Gas volume and pressure both work the comp. High pressure can work as well as lots of gas but more wear and tear on the gun.

[rishii]

Not a true apples to apples comparison 

I used to do that when I ran a supercomp open gun, 115 blues for local matches, 124 jhp for majors

same powder charge and pretty much the same PF

Dot movement was about the same and I could switch back and forth without any issue

 

[lroy]

19 minutes ago, Brazos Custom said:

To compare you would want the case volume when the bullet is seated to be the same, not the same OAL. Shorter bullet would need to be seated further in. Pressure peaks in the first couple tenths of an inch. Small changes in the amount of case volume can mean big pressure spikes. That's why people worry about setback. Gas volume and pressure both work the comp. High pressure can work as well as lots of gas but more wear and tear on the gun.

 

so the 115 seated to the same length as the 124 would have more space in the case resulting in less pressure and the same amount of gas?

[Brazos Custom]

Pretty close. Still a few variables  depending on burn rate, barrel, etc.,but pretty close.

[shred]

3 hours ago, lroy said:

For example, that test where you fire against a no shoot and see how much splatter comes out the front of the barrel. Theoretically, there is a maximum amount of gas the comp can handle and you want to achieve that point and no more, correct? So if you reach that point with a heavier bullet, is there no reason to try a lighter one, as the excess would only exit the front of the barrel and hit your hand?

There is no 'maximum amount' of gas the comp can handle.  That just tells you what powder granules and whatever come flying out of the muzzle.  Push more through it and more comes out the front but some is still going to be redirected.  You can also wrap electrical tape around various ports on your comp to see how may wraps each port breaks.

 

All told I'd bet nearly none of the top shooters bother to do either.  They work up a load based on PF, bullet and powder and go from there based on what the dot does versus what they want it to do and leave the comp and port design largely up to their gunsmith.  A few of them will drill extra holes and saw off ports and so on to experiment with what that does to the dot motion.  

 

There is no 'best' for everyone, it's a personal thing.  Max kicked everyone's ass for years in Open with 125gr bullets and <8-ish grains of 7625 in essentially a STI Trubor.  Other dudes do real well with hybrids rocking 10+gr of N105 or 3N38 and 115gr bullets.

[zzt]

When you go up in powder charge the gun shoots softer and softer (and flatter)  right up until the poppels (if any) and the comp cannot redirect any more gas.  Once you push past that point, recoil increases.  It increases because the excess gas is jetting out the front of the comp.

 

As mentioned above, it is different for each gun.  For instance, my main 9mm Open gun wears a very efficient comp and two 3/16" poppels.  I literally cannot fit enough AA7 in the case to exceed that combo's capability.

[lroy]

maybe that's where I'm getting confused. I was thinking more gas = less dot movement, but you're saying you can use too much and start increasing it again?

Edited April 7, 2023 by lroy

[zzt]

12 hours ago, lroy said:

maybe that's where I'm getting confused. I was thinking more gas = less dot movement, but you're saying you can use too much and start increasing it again?

 

Not exactly.  More gas past a certain point will increase recoil and slide speed.  It will hit your hand harder.  You may be willing to put up with that if the gun shoots flatter. 

 

If your goal is to have the softest shooting gun, stop when gas starts jetting out the front.  If your goal is the flattest possible gun, go hotter and live with the increased recoil.

 

If I was going for flatter, I'd rather add more poppels.  They are much more effective at minimizing muzzle rise than the comp is.  Then you can add more powder to make up the gas the extra poppel(s) robbed from the comp.

Edited April 8, 2023 by zzt