I went through all of my notes. I don't have any data on the port size for a 6" barrel. Smallest I have is 8" with a 0.76 diameter port.
I suspect that's a typo, 0.76 is an awful big port. Not gonna be much barrel left.
I didn't know I needed 300bo specific stuff to build one. I thought it was supposed to be a barrel change only kind of caliber. At least I now know what to look forward to when I finally get a chance to build mine although I do have the suppressor for it already.
Don't forget to post the results and what fixed it I am keeping an eye on the thread for future reference.
If you do plan to go with a lightweight BCG I would suggest getting a decent adj gas block to go with it. I have a superlative arms bleed style block on my lightweight 556 gun, and with it properly tuned the lwgt bcg and 1 oz buffer make the gun very nice to shoot. As the reciprocating mass is much less the gun does not move around much when cycling.
You can remove the guts of the buffer as that will reduce the mass and possibly help with the undergassed situation.
As mentioned, an adj gas block will not cure an undergassed gun (only opening the port will fix that) but with the adj gas block and a lighter bcg and or buffer you can tune the gun. The lighter power recoil spring looks like the easiest thing to do, gutting a buffer will be the cheapest, and dropping 200 bucks on a lwgt bcg, 10z buffer and an adj gas block will give you a bunch of tuning potential.
When thinking of running such a wide spread of ammo velocities (300blk most have the widest spread of velocities of any caliber I can think of), an adj gas system is a must. I like the lwgt bcg as it offers smoother shooting.
If you're undergassed while shooting subsonic loads unsuppressed, I would hold off on drilling the gas port until you actually get to shoot subsonic suppressed.
The last thing you want to do is open up the port, then add an adjustable gas block to remedy the fact that you opened up the port.
Off the top of your head.....do you remember how much the lwgt bcg weighs?
maybe 5 oz... is the stock 8oz? For some reason those numbers stick in my head.
The JP Low Mass BCG is around 8-9 oz. If you have a BCG that weighs 5 oz, that can lead to a lot of issues.
And since springs were brought up previously, this is worth looking into. Many people with 300BLK AR's running low pressure ammunition have found them to be beneficial.
http://www.davidtubb.com/ar15-buffer-spring-stainless
.076in, somebody just moved the decimal. The older colts are a tad over .06 and the max is a little over .08 inchesI suspect that's a typo, 0.76 is an awful big port. Not gonna be much barrel left.
Weigh your buffer on an oz scale, if you have dial calipers or access to some you can measure gas port also since you dont have gauges. Tuning for optimization can start with gun but usually comes down to load development also. There are known port sizes that work with pistol lengths and an adjustable gas block can help with both over and under gassed guns as it loosens or restricts gas flow, according to science and extensive testing that is
My buffer is 3oz.
You have the equivalent of an H1 buffer.
Dropping a carbine buffer in and a Tubbs spring should solve your issue.
Thanks for all of the info.
I have been kicking around the idea of possibly getting a JP Silent Captured Spring Buffer set for this. I think I can get them with the different springs in a kit. I had one of these in an old rifle at one time and it was very nice to shoot. It was a rifle that was already built when I got it so I don't know which one was in it and I only had the rifle a short period but have wanted another JP buffer for a while.
I am on the fence about ordering just the Tubbs spring for what I have or the JP kit.
I would avoid the JP Silent Capture Spring. It’s known to cause more trouble than it’s worth.
I’ve got 3 of them in different rifles. One in my best 300. The 3 spring AR 15 or 5.56 kit is the one in my 300 and I used the mid weight spring. I’ve never had any problems with these. Can’t speak for anyone else’s experience but I haven’t heard of any issues either. They’re a little cheaper now than the ones I bought a few years back.Really? It felt so nice on the rifle I had for the short time I had it. Didn't know it was giving people problems.