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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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Discussion Starter · #13 ·
Did it make any difference when you played with the gas setting, or is it at the point that you cannot change it anymore?
Update:

Using the same ammo referenced above (fast burning powder), the Bren 11" barrel produces the same velocity as it did with the gas port completely closed (single shot mode).

This tells me that it isn't dumping a volume of gas sufficient to impact muzzle velocity. Not sure why I didn't try this in the first place, but here we are. My conclusion is that the barrels are simply that much different.

I cannot comprehend than an ARP 12.5" barrel is producing 350 fps more than an 11" Bren and only 100 fps less than a 16" Ballistic Advantage barrel...but it has been proven every time I go to the range.

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Update:

Using the same ammo referenced above (fast burning powder), the Bren 11" barrel produces the same velocity as it did with the gas port completely closed (single shot mode).

This tells me that it isn't dumping a volume of gas sufficient to impact muzzle velocity. Not sure why I didn't try this in the first place, but here we are. My conclusion is that the barrels are simply that much different.

I cannot comprehend than an ARP 12.5" barrel is producing 350 fps more than an 11" Bren and only 100 fps less than a 16" Ballistic Advantage barrel...but it has been proven every time I go to the range.

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The same thing has been noticed when comparing ARP 6.8 barrels with others . In the 5.56 world you have . 223 .223w and 5.56 chambers standard . 223 chambers are too short ( bullet to lands ) for full throttle loads 5.56 are long enough for full throttle with longer heavier bullets . .223 w are just right to allow plenty of bullet jump for most bullets to keep pressure down to manageable levels yet develop pressure for velocity with most bullets . You have the distance to lands in the ARP that is best suited for your bullet plus the benefit of superior land/ groove design . My experience has been that powder burn rate is tied more to bullet weight than barrel length . The powder that gives the most velocity with my 18" gives the most with my 12.5 " .

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You may find that the ARP barrel is a bit more than 2" longer than the other barrel. And, at some point the velocity vs length becomes very unlinear with length so there is a steeper fall off with the shorter barrel. Then add the chamber difference, land variation, etc. and it may add up to the difference you are seeing.
 

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Discussion Starter · #16 ·
This entire experiment began as an exercise to understand the effects of burn rates as they relate to barrel length. If the fast burning powder gives me more velocity in a 12", why wouldn't I use that for all guns with a similar chamber and gas system length?

I will perform the same experiment using 69gr and 77gr bullets to see what that tells me.

H does say to use 13" as the barrel length for NFA purposes, but I'm still in shock that there's a 350 fps difference...especially considering the 556 chamber on the 16" barrel.

The goal here is to have a rifle that has sufficient velocity as to limit overpenetration (due to loss of velocity) for self defense rounds. Yes, SD ranges are generally close enough to not have to worry about sufficient velocity, but it'd be nice to know that overpenetration is minimized beyond 100 yards.

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This entire experiment began as an exercise to understand the effects of burn rates as they relate to barrel length. If the fast burning powder gives me more velocity in a 12", why wouldn't I use that for all guns with a similar chamber and gas system length?

I will perform the same experiment using 69gr and 77gr bullets to see what that tells me.

H does say to use 13" as the barrel length for NFA purposes, but I'm still in shock that there's a 350 fps difference...especially considering the 556 chamber on the 16" barrel.

The goal here is to have a rifle that has sufficient velocity as to limit overpenetration (due to loss of velocity) for self defense rounds. Yes, SD ranges are generally close enough to not have to worry about sufficient velocity, but it'd be nice to know that overpenetration is minimized beyond 100 yards.

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You are going the wrong direction trying heavier bullets . For less penitration and higher velocity use something like 50 grain so or v max . No matter what powder you use hp or solids are not going to be fast enough out of short barrel to reliably fragment . No matter what powder you use the same load is going to produce less pressure in a 5.56 than a . 223 w hence less velocity .

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Discussion Starter · #18 ·
You are going the wrong direction trying heavier bullets . For less penitration and higher velocity use something like 50 grain so or v max . No matter what powder you use hp or solids are not going to be fast enough out of short barrel to reliably fragment . No matter what powder you use the same load is going to produce less pressure in a 5.56 than a . 223 w hence less velocity .

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My previous message was out of order. The goal with the heavier bullets is experimentation to better understand barrel differences, not necessarily to use them for SD. Thank you for pointing out the issue with using them in an SBR. However, I do get almost as much velocity in the 12.5" ARP 223 Wylde Mk1 as other guys get from their 16" 556 guns, when testing side by side.

It seems like you think it's plausible that the velocity difference can be explained by the combination of chamber pressure and rifling/bore differences. Idk what else it could be, so that's why I asked here. Thank you.

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My previous message was out of order. The goal with the heavier bullets is experimentation to better understand barrel differences, not necessarily to use them for SD. Thank you for pointing out the issue with using them in an SBR. However, I do get almost as much velocity in the 12.5" ARP 223 Wylde Mk1 as other guys get from their 16" 556 guns, when testing side by side.

It seems like you think it's plausible that the velocity difference can be explained by the combination of chamber pressure and rifling/bore differences. Idk what else it could be, so that's why I asked here. Thank you.

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If you have not settled on which bullet to use try the sierra 65 grain game king . It has the expansion and penitration I prefer and is accurate in my guns .

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Discussion Starter · #20 ·
I plan to stay 55gr and under so that my hold over is more familiar since I train with XM193. I have a ton of 50gr VMax, and I'm not preparing for barrier-blind encounters.

Any insights toward answering my other question?

If the fast burning powder gives me more velocity in a 12", why wouldn't I use that in longer barrels with a similar chamber and gas system length?

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