Tuning a 20" rifle length gas system.

[PGW Steve]

I posted a thread before about the accuracy I'm getting out of my 'Canadian' rifle, manually cycled.

When I ordered my barrel, I asked the barrel maker to leave the gas port undersized so I could tune the system with a JP LMOS carrier. As delivered, the barrel came with a .086" port, and from what I could determine on this site is that a .0935" port is the norm for a 20" RL system. I'm using a low profile DPMS gas block and it is perfectly aligned (verified with bore scope). No suppressor is being used.

With the .086", the rifle wouldn't grab a second round, or hold open.

I drilled it to .089" (next size up in number drills) and it would sometimes be held by the carrier on the bolt stop, but not grab another round.

Just now, I opened it up to .0935" and it's picking up rounds, but only holding via the carrier and not the bolt head.

I've tested with Federal GMM 130 as well as some handloads. I thought the low mass carrier would make it fully cycle with a smaller port than normal, or am I thinking about this in the wrong direction? I don't have a full mass carrier to test with right now.

Should I just go up one more drill size and get it to lock back?

[Klem]

Steve,

It would seem that a slightly wider gas port is the solution. The norm is drilling a generous port and then choking the excess gas with an adjustable gas block or choosing different buffer weights to suit.

An AR15 rifle buffer is typical 142grams in weight which is equivalent to a H3 folding stock/carbine buffer. Carbine buffers come in 5 stock weights for tuning a gas system. If you are keen on squeezing the most accuracy out of your receiver (which is what I understand you want to do from your other thread) then it lends itself to a fixed stock, something that has absolutely no play in it. The Restricted category probably makes a fixed stock compulsory anyway. That means you are stuck with a 142gram buffer (carbine and rifle buffers/springs are not interchangeable) so you need to tune the gas system some other way. The other tuneable parts of the system are locating the port somewhere else on the time/pressure curve (less flexible than it implies due to standard lengths of gas tubes and the need for optimum dwell time), adjusting gas at the carrier or block, adjusting the weight of the carrier or buffer, different strengths of recoil spring, or drilling different diameters at the port. It sounds like the next wider hole drilled in the port might be perfect.

Your light-weight carrier can be part of the solution but that comes at a price of having less momentum to pick up the next round. If your mag springs are not gripping those rounds too tightly when they are fully compressed and you're not wading through mud then it's probably a non-issue.

Choosing the powder carefully would be another way. A powder that has more port-pressure as the bullet passes the port. The downside of that is it will limit your choice of powders and potential muzzle velocities. In this calibre we don't have that luxury as we squeeze every fps we can out of a relatively small boiler room. It's a clumsier solution to what you are currently doing but just putting all options on the table.

I would drill the port wider. Sounds like you're doing it in careful increments so it's not going to hurt anything except rob the muzzle velocity of a few fps. If it ends up too wide (i.e. too much gas) then you can always source an adjustable gas block and choke it back to what works.

[PGW Steve]

Wow, thank you Klem!!!

I do want to pick up an AGB, but getting a good one in Canada is a chore. Considering I know my way around a machine shop, the drilling process isn't too much of a problem. I was initially concerned when I ordered the JP Low Mass carrier, and didn't want to be grossly over gassed. I'm partially curious if anyone is running a non adjustable system with a larger than .0935" port? The next number size is a pretty small jump, only .0025", unlike the .0045" on my last step. I'm glad I did it this way and know I'm not super over gassed and beating the snot out of stuff.

Restricted class doesn't mean fixed butt stock, we can have collapsible provided the OAL is greater than 26", which with a required (on autoloaders) 18.5" barrel isn't difficult.

Since we are going into a deep freeze right now, I'm not in a hurry to get this tuned. I picked up a Magpul G3 PRS stock which has the parts to work on a carbine tube, I'll pick up the associated parts and tune with weight. This is a learning experience for me, so I like seeing how small increments affect the whole system. Tinkering with buffer weights would be simple enough with a measureable result.

We're limited to magazines riveted to 5 rounds, so there is next to no spring pressure. A loophole allows us to use 'pistol' magazines with 10 shot capacity, God Bless you guys for .223 and .308 'pistols'!!! I have a pile of CPD .223 and 7.62x39 ten shot mags, these don't seem to tax my thumbs like a full 30 does.

Again, excellent info, thank you! I'm going to get a carbine tube/spring/buffer and work on tuning that way. If I can get a decent AGB, I'll go larger on the port down the road. If the SHTF tonight, I'll grab the Dewalt and a #41 drill LOL

[LRRPF52]

20" RLGS/.0935"/standard M16 mass carrier/any buffer is an easy configuration out of the gate for reliability.