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20 GT AI Overbore Powder Selection

ZenArchery

Got the 20 GT AI out on its second run with fully fire formed cases.
I was predicting, expecting 4300FPS range but ran into pressure around 3800-3900FPS.
After doing some searching I've come to the conclusion the H4350, WinStab 6.5 is too slow. CFE-223 got me a little closer but the primer gave out before showing case pressure and failed to cycle bolt. Meaning if this was a bolt gun and not an AR15 it would work just fine.

I have had a lot of luck with CCI 450M with other wildcats I've run. But waiting for my brick of primers to arrive before I can do my next batch of test.

I spent an hour with GRT and QL and I've narrowed it down to 3 powders SW Precision, Ram Hunter, and Win 748 to give me FPS I am predicting. The fully fire formed case hods 45.5gr of H20 gr of water weight. Benchmark, H335, and IMR3031 got me the FPS that I'm predicting but most of these burnout in the first 9-13" of the barrel not allowing enough back gas flow to cycle the bolt. (CFE-223 burned out at 13").

The accuracy of this shooting system at 3550fps is mind blowing but this is my blow out the case to 40º shoulder load.

My next step is to test the different powders. Apply a crimp for potentially more pressure.
This is a fun project. Hopefully I can find a combo that works before burning the barrel our at predictably around 600-700 rounds.

 
powders somewhere around the RL15 burn rate are better suited to your case to get fill ratio -…

That primer thing isn’t an AR thing either- it’ll happen in bolt, just as easy.. ;)

VV135,140-150 may be the ticket…ball powders peak in temp swings or loaded rd left in chamber for 15sec to heat the rd up..

I’m using N135 in 20BRAI-55gr bullets @ 3750fps..
 
Tommie said:
Low velocity with pressure signs and you want to use faster burn rate?
I’m confused.
Click to expand...
I appreciate ZenArchery sharing these results. I’m assuming that this is with the 55gn Berger?

It’s always interesting to see new things that are off the beaten path, but the chosen burn rates do not make sense to me, nor do the comments on the powder burning out too quickly to cycle the bolt.

I would think that if H4350 and Staball didn’t give enough speed before pressuring out, then a slower powder is called for, provided that there is room in the case. Unfortunately, with huge overbore cartridges, I’ve got to places where more powder stops giving me more speed regardless of what I do. I’d give RL-17 a try if you have it. RL-17 has always given me another gear and usually shoots quite well if you’re willing to manage the downsides.
 
Tommie said:
Low velocity with pressure signs and you want to use faster burn rate?
I’m confused.
Click to expand...
Not easy to obtain but I'd bet RL26 would be a top performer in something like this. Most of those other powders seem like they'd be way to fast for something this overbore.
 
I have run my 20BRAI with 1/10 twist barrel and BIB 40gr bullets close to where you want to be. With AR Comp I've been up to 4200fps, have a mild 4000 fps load with RL 15, and got some VV140 to try as well.

Surprised no one has suggested VARGET. I bet N540 would get you speed at the cost of excessive heat from the energy content.
 
TroyG said:
I have run my 20BRAI with 1/10 twist barrel and BIB 40gr bullets close to where you want to be. With AR Comp I've been up to 4200fps, have a mild 4000 fps load with RL 15, and got some VV140 to try as well.

Surprised no one has suggested VARGET. I bet N540 would get you speed at the cost of excessive heat from the energy content.
Click to expand...
Did you ever try the n140?
 
I tested some 46gr bullets in my 20BRA at 4050fps shot really well there never did go any higher to see were top end was.
 
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