Build Thread Converting AR15 300aac pistol to piston

[jassing]

I wanted to use Superlative Arms conversion kit for an ar15 pistol in 300aac; but their kit doesn't support 300aac.
That leaves the Adam Arms kit. Before I go that route, I was wondering if anyone tried the superlative arms and got it working on a 300aac pistol?
Or is there another option? (other than leaving it as DI)
thanks

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[Member 34319]

Why wouldn't it support 300 blackout. If the piston length is the same as the gas tubes, wouldn't it? You could always drill out the gas port if it has problems cycling.

 

[jassing]

They said it was a pressure issue. (I thought 223 & 300aac were the same pressure; but what do I know...)
on their website it says NOT RATED FOR 300 BLK!!!

So, I asked - they said "lower pressure"
Adam Arms has a kit specifically for 300aac. It seems to use a shared gas block, so it's got to be the spring is the difference... ?

So there must be something... which is why I asked, to see if anyone else has used it with 300aac and found it worked or got it to work with some other spring.

 

[Member 34319]

I know what they are referring to.

Carbine length gas tube on a 556 16" barrel is something like 25,000 psi at the gas port. Go mid length gas tube on the same barrel length, pressure reduces to about 20,000 psi. That would be the pressure operating a piston if converted.

300 blackout has similar chamber pressures as 223/556 that's why it can use the same bolts as 223/556.

I have no freaking clue what it's average pressures are at the gas ports are at various gas lengths. I'm going to say, likely less as most 300 blackout barrels tend to have short gas lengths, which leaves me thinking the gas port needs to be closer to the chamber to get adequate pressure to operate the action. Which also means the operating pressures further down the barrel are likely significantly less.

If anything maybe 300 blackout has a more dramatic pressure curve.

If your into tinkering with things, I don't see how you couldn't not make it to work.

 

[jassing]

well, that's a much better explanation of it than they gave me. They just said "the lower pressure".
What I don't get tho is that pistol length is pistol length whether it be 223 or 300. DI or piston systems would send the same pressure back to operate the bolt (not the same for 223 & 300 but 223 DI v piston; and 300 DI v piston)... I don't fully understand it all; so it's sort of magic to me. To me, the piston seems an extension of the bolt carrier, and the main difference is the gas hits either the carrier or the extension. I don't know if I've explained it well -- like I said "magic to me"... Maybe the difference is in the timing (how fast the gas acts on the carrier).

I was given a basket case AK where it didn't cycle well at all. finally had an AK guy look at it and he said "It's your gas timing" and had to get an adjustable gas block because for some reason, someone had drilled out the barrel's gas port which messed everything up.

I am tempted to try it, but I don't know where to find other springs to try. I'd be a lot more motivated if someone said "Oh yea I did that; I got springs from [place] but I forget which one" -- at least I know I could succeed.
But, I'm a whimp, so I'll just go with the Adams since they already worked it all out.

Thanks.

 

[Member 34319]

Their pressure curves are much different.

Peak pressure on a subsonic 300 blackout (which uses similar powders as magnum handguns) may be similar to 223/556, but by the time the bullet is near the muzzle the pressures could have dropped 80-90% for all I know, which I'm not too familiar with. Hence why barrels in 300 blk have shorter gas systems. They need that higher pressure from closer to the chamber to operate the action.

As the bullet travels down the barrel, the pressures drop. Some cartridges this happens much faster than others.

A supersonic 110 grain 300 blackout load might be using a powder that burns much slower than a 220 grain 300 blackout load. Both will be drastically different than a load for 223/556.

Like a hose, the more pressure you have combined with a smaller hole at the end produces a pretty good stream. Open up the hole at the end with the same pressure and you get a ton of water all at once. Same thing applies to a gas port on a barrel. Open up the gas port, you get more pressure, faster. With that said, I don't see why, with enough toinkering, you couldn't get it to work.

 

[jassing]

I don't see why, with enough toinkering, you couldn't get it to work.

<warning: I have only a cursory understanding of the dynamics going on with a DI vs Piston>

Won't get into pressure vs volume but my point is still the same: gas pressure goes up the barrel, thru the gas block, down a narrow tube to act on the bolt carrier. The only thing that's different is the timing - the gas travels less before it meets a solid object, so going on this theory, a piston system would act on the carrier too soon, unless it was delayed, so it would need a smaller port so gas pressure takes longer to build.

If SA's kit can't "dial down" the adjustable block enough, then it can't be made to work. Maybe a stronger spring? Longer? Where to source them? Maybe it's shorter? or weaker? I dunno. Which is why I'll go the AA this time.

WA law changes next week, stripped lowers will no longer be sold as "other", so as soon as I find one close by, I'll grab it may do another pistol project. I'm willing to take donations for the SA and give it a try... ;-)