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Discussion Starter #1
This is mostly a copy/paste of a post I just made on the CZ forums.

My SBR AMMO (IMR 4198) runs approximately 350fps slower out of an 11" Bren 2Ms than it does out of a 12.5" AR15.

How can the difference be so great? Am I wrong to be surprised by this much of a difference?

I expected MAYBE 100fps different.

Hesitant to make a post about this, but I couldn't readily find anything online specific to my question.

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I know the 3R rifling helps with speed compared to standard 6 groove. I've never heard of a huge velocity difference from it but is your bren overgassed to the point that is is dumping enough gas to affect bullet speed?
 

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I suggest you load some rounds with a similar burn-rate powder and see if the result is similar.
 

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Discussion Starter #4
These are the reasons that I posted this here. I was thinking about the rifling, the type of rifling, and the surface of the bore as well. It's definitely over gassed, but I hadn't thought about it shedding that much gas.

If it's letting that much gas through, it almost seems like it's not possible to optimize ammunition for this gun because that would imply the gas block limits the pressure behind the bullet.

With a slower-burning powder, I don't recall the difference being so significant. Maybe it is dumping gas. I'll have to go look at the numbers for the other rounds.

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Did it make any difference when you played with the gas setting, or is it at the point that you cannot change it anymore?
 

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Check your distance to the lands on both barrels. I had an 18" that shot the same load the same or faster than a 20" which had 0.035" more freebore.
 
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Discussion Starter #7
Did it make any difference when you played with the gas setting, or is it at the point that you cannot change it anymore?
Good idea there...It would be worth running it with the gas system closed in order to see if that increases velocity.

The Bren was on setting 1. Setting 2 would send additional gas to the piston. Also, I didn't notice any obvious signs that the bolt opened early.

I'm trying to visualize how the bleed off works, but I'm not familiar enough with the Bren system.


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Multiple factors stacking up . It is possible arp has shorter freebore "= more pressure = more velosisity and less gas bleed off = more velosisity and less bullet engraving = more velosisity and the obvious 1 1/2" longer barrel. Also are both guns being tested with cases formed to their chambers or new cases ? If not fire formed and equal shoulder bump that can effect pressure = velosisity + or - . It is kinda like ounces = pounds , a little here and there in the same direction adds up quickly .

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Discussion Starter #9
All of these cases are from the same source and sized the same, not to fit either gun.

You think the more generous CZ chamber is allowing greater case expansion and therefore further decreased pressure?

It seems that the CZ chamber is larger...there are cases that won't chamber in the ARP (because I messed up a crimp) but the CZ chews through them.

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All of these cases are from the same source and sized the same, not to fit either gun.

You think the more generous CZ chamber is allowing greater case expansion and therefore further decreased pressure?

It seems that the CZ chamber is larger...there are cases that won't chamber in the ARP (because I messed up a crimp) but the CZ chews through them.

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Not just the chamber being larger but all of the differences I listed before . The safest thing to do would be start over and load a round tailored to each individual chamber . The first step would be to do as has been previously suggested and check the difference in the chambers by checking the shoulders.

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and distance to the lands.
 
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