What gas port size for 20" 450 Bushmaster?

[isptroop38]

My buddy has a 20" 450BM upper that is undergassed. I can open up the gas port, but I can't find anything on the proper gas port size for a 20" barrel. I have found lots of info on the 16" barrel, but nothing on the 20" with rifle length gas system. Anybody know the correct size?

Thanks in advance...

[Hoot]

My buddy has a 20" 450BM upper that is undergassed. I can open up the gas port, but I can't find anything on the proper gas port size for a 20" barrel. I have found lots of info on the 16" barrel, but nothing on the 20" with rifle length gas system. Anybody know the correct size?

Thanks in advance...

First off, what gas system length is he running on that 20" barrel? The further from the chamber, the larger thge port needed. I had to open a rifle length port up to .108 to get it to reliably cycle lighter loads. From my reading and persona experience is that the mid-length seems to be the best compromise for a 20 incher. Drop down to a 16" carbine barrel and it is best served by the gas system length of the same name. That close to the chamber, the pressure is still pretty high so a smaller port size suffices. I don't have my stock Bushmaster barrel anymore, so I can't say what the port size was that they chose but I know that they used the same carbine length system for both their 16" and 20". I can say my 20 incher digested almost everything I ran through it.
Given your friend is having trouble with his stock Bushmaster 20" barrel, I'd do some serious investigating before opening up that port. The one I had ran fine right out of the box without needing break-in and I even added weight to my bolt to go easier on the brass since I reload. I did not run mine particularly wet but I did burnish in Dow Corning MolyKote G-N MoS2 bearing grease on every surface that dragged on one another during cycling. I also cleaned the rifle after every range session, usually while it was still warm. By that, I mainly mean the bore, bolt and BCG inside where carbon builds up. I didn't hose the entire action down with any kind of solvent as I didn't want to efface the moly grease and have to reapply it. I did put some form of oil in the bolt hole before each use. Read not WD-40, not 3 in 1 oil!!! a Quality CLP and later some Slip 2000. Now I use Sprinco MGL moly bearing oil to complement the moly grease. A great 1-2 Punch.

Opening up the gas port should be your last resort for cycling issues, especially on a carbine length gas system like Bushmaster uses.

Hoot

[isptroop38]

Thanks Hoot, I know you have great knowledge in this stuff. It's not a factory Bushy, it's a Radical Firearms. The gas system is rifle gas length.

[Hoot]

Thanks Hoot, I know you have great knowledge in this stuff. It's not a factory Bushy, it's a Radical Firearms. The gas system is rifle gas length.

That's not the best news to hear as again, I struggled getting my rifle length system to run reliably. It originally came with a .078 port diameter which told me the manufacturer didn't think it through. Too bad as it was machined with great attention to detail otherwise, with a nice black nitrided finish. Using a set of sharp numerical bits, I walked it up one bit increment at a time (very time consuming) pausing to run to the range and test it. I'm fortunate to have a rifle range less than 10 minutes away that I'm the manager of. Even then, it got real old and (don't cringe) I finally broke down and took the numerical bit set, cutting oil, drill alignment jig and a variable speed hand drill to the range and proceeded to drill, clean, shoot, repeat. I used my 200gr FTX over Lil Gun load recipe as that's as light as I go, to determine success approaching. When I got to what I thought was a decisive almost there point. I went back home, set up my press for a clean cut and performed the final port enlargement. I followed that with a bore lap procedure to remove any burrs that might be present as at that time, I did not have a bore scope to examine it. That port diameter wound up being .1065 and This is Important: I was in direct contact with Tim LeGendre (Wildcatter) at the time as he was pushing his Corvette concept. He confided that he had to go as high as .120 depending upon how well the barrel and action was broken in. My barrel was new at the time though my test action was well broken in and lubricated as previously mentioned.The Point Being: There is no one size fits all for this process. Too many variables. At least if you don't want to overshoot the port size appropriate for your setup and then having to resort to an adjustable gas block. I'm a firm believer in K.I.S.S.
That .1065, combined with my broken in action, reliably cycled all my loads without the perception of being over-gassed. The bad news is, at that point, I was disgruntled at having to put in so much work just getting that barrel to work and was sick of looking at it. I sold it to get it out of my sight.

Hoot

[isptroop38]

Understood, thank you very much sir!

[Bmt85]

Hoots right about "one size doesn't fit all". A lot of other things factor into the equation.

I have a build using a BHW 20" rifle gas barrel, can't remember what the port size is (probably .120), using a side charging upper, with a Melonite bcg, and for fun ran a Slash's XH buffer. A 225 FTX with 39gr of Lilgun (that's a pretty light load), and Hornady FTX factory ammo have just enough gas to cycle the action. Running a standard carbine weight buffer (3oz vs 8.5oz for the XH) will run better, using a Nickle Boron carrier will help too.

So you should be able to get that upper running, might have to play around with a few things to get there, but it should work.