Does anyone or can anyone make a custom bbl for a 20” 6.5 Grendel build. I want a light (#4) but I need it contoured so the last finished 1” will be big enough to thread 5/8x24 and leave a shoulder. I prefer a cut rifled bbl just because. Thanks!!
Gunsmithing Custom bbl for muzzle threading
- Thread starter krw
- Start date
A 5/8-24 thread has a major diameter of .625. Any barrel that’s roughly .725 and up at the muzzle will be sufficient.
Just a FYI. Any cut barrel maker will make just about any contour you like at no extra charge as long as you stay under their breech diameter and length limits. Usually that cutoff is over 1.25” diameter, and over 28” finish length. You can go bigger and longer, but you’ll pay extra for it.
Just a FYI. Any cut barrel maker will make just about any contour you like at no extra charge as long as you stay under their breech diameter and length limits. Usually that cutoff is over 1.25” diameter, and over 28” finish length. You can go bigger and longer, but you’ll pay extra for it.
Interesting idea.
i recently saw a video where the smith demonstrated (with gage pins) that threading with "minimal" barrel wall thickness resulted in the bore "swelling" at the tenon.
This concept would mitigate that, though i gotta wonder how that affects harmonics.
i recently saw a video where the smith demonstrated (with gage pins) that threading with "minimal" barrel wall thickness resulted in the bore "swelling" at the tenon.
This concept would mitigate that, though i gotta wonder how that affects harmonics.
This only occurs in poorly stress relieved BUTTON rifled barrels. Cut rifled barrels and quality button barrels do not exhibit this. You can cut the wall thickness down to 0.1" or less (if you so choose) and the bore will not swell.
In poorly stress relieved button barrels where this does happen, you have to keep the thread diameter thicker, like a 3/4-24 thread to keep it from happening. The only time I've ever seen it happen personally was with a gunsmith special Green Mountain blank. I profiled the barrel myself from a 1.25" blank then cut muzzle threads and a .305 gauge pin fit in the muzzle about 1/16", and a .301 would go about 1/2" into the muzzle end, where a .300 was a slip fit down the rest of the bore (as far as I could push it in and still pull it back).
Tried the gauge pin trick on a threaded Benchmark button barrel and a .301 would not go in, .300 was a slip fit. Likewise in all brands of cut rifled barrels I had available at the time (Kreiger, Bartlein, Brux, Rock Creek). Also saw the video online of someone checking some factory grade AR-10 barrels that exhibited bell-mouth behavior.
Recently I set up an excel spreadsheet and fed it in bore pressure info from a couple curves I found online that give bore PSI vs. length of barrel. Then I analyzed the hoop and radial stress and calculated the expansion of the bore given 416R stainless material. I was looking to see how much the bore stretched radially and basically what I found is that as long as your barrel is over 18" long, the wall thickness at the muzzle doesn't matter as far as bore expansion goes. Basically from about 4" down the bore (peak pressure) the expansion (even on a 1.25" straight bar) is ~.001" +/- a couple tenths, and by the time you get 18" down the bore pressure drops off enough that wall thickness could be thinner than the smallest available profile and still not expand more than .0002" or so.
Up next I'm going to mess with thermal expansion effects added to that ^^^ and would also like to look into muzzle displacement and stiffness for a given profile.
I've never seen it done, but I imagine you can silver solder on a bushing/sleeve on the muzzle and turn/thread/cut it to be concentric to the bore with a shoulder for the can. I've also seen brake-attached suppressors attached with a jam-nut and loctited in place where the barrel end was not of sufficient diameter to cut a shoulder.
