You may want to call adco. {not sure if this will be what you are looking for}
http://www.adcofirearms.com/shopserv...e%20Gas%20Port
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You may want to call adco. {not sure if this will be what you are looking for}
http://www.adcofirearms.com/shopserv...e%20Gas%20Port
Clint,
You wouldn't happen to have or know where I can get prints/drawing of the bbl dimensions for Mid or the Int length barrels? I work off a print from an M-4 barrel contractor, so it has all the dimensions spot on.
I will also post additional info on timing the extension a little later.
Opsoff
Check your private messages...
This is honestly one of the coolest threads I've ever read. I weld on the civilian side, and picked up a love for tinkering/gunsmithing somewhere along the way. While I'll probably never have such an extensive shop as opsoff1, it is awesome seeing what a person can do if they're willing to learn!
Timing the gas port - as promised,
A very condensed version without getting into all the minute measurements and boring details.
The first and important layout begins with the 20" bbl set up in the milling machine where the bbl is indexed off the existing gas port with gauge pins. From this, a witness mark is machined (center drill pointed) at the location on the bbl diameter just ahead of the shoulder.
After the length layouts have been transferred to the 20" bbl with Dykem/scribe marks, the bbl is set up in the lathe and adjusted with bore rods to show less than .0005 TIR - basically the bbl is turning on the exact center of the bore. The chamber end is cut off with a parting tool on the lathe. Two critical measurements are adhered to - one is the length to the shoulder and the other is the length of the bbl tenon (threaded shank) The exact measurement to the shoulder is left .0625" long to account for one revolution of the 16 pitch threads. (.8125 x 16UNF) The tenon is left about .100" long. Once the threads are cut, the extension is threaded on the tenon and the alignment of the extension index pin is measured radially to determine how many degrees out the pin is in relation to the witness mark. Once this is known - basically a .0017" cut will move the bbl extension 10 degrees.
It is rare that an extension is out by more than 120 degrees. Most are in the 45-90 degree range.
Lets assume the extension is out 120 degrees - this would necessitate a cut of about .020" (120/10 = 12, 12 x .0017 = .0204) This would theoretically put the index pin in exact alignment with the witness mark on the bbl.
Tests on cut off shanks determined that I need to leave the index pin about 5 degrees out from alignment on a chrome moly bbl vs about 7 or 8 degrees on a 416R stainless bbl in order to gain the propper torque value when I install the extension (150ftlbs).
So in the end - I end up with an extension that is timed exactly to the gas port - technically the measurement to the shoulder would end up being .040 long. This is truly meaningless as gas tubes and other components vary more than this. An initial install where the extension is out by 3/4 of a revolution would end up being long by about .015" Meaningless.
Lastly for the naysayers - being .020, .030, .040 long only gives you a tad more clearance in the area between the FSH and a free float tube or rail system.
Once the extension is timed and before torquing (yes I use Loctite as well), measurements are taken and the tenon is faced back to the nominal dimmension of .750" long. This is adjusted a few thousandths (+/-) to account for bolt lug clearance - usually >.005. Larger gaps can induce bolt bounce.
It is at this time where I chamber - usually in .223 Wylde - eats everything with great accuracy. Chambers are cut to minimum headspace with the component bolt: 1.464" To account for shoulder crush, an added .002 is cut in the chamber, when the extension is torqued on, the head space comes right back to min. spec.
I have done easily 40+ M4 bbls this way and everyone has been trouble free and had amazing accuracy.
What are your questions.
Thanks for sharing some excellent info! I have a big, fat SS match barrel that I might try this on.
Two questions: What is the difference between "the length to the shoulder" and "the length of the barrel tenon?" Aren't these the same thing or are you referring to a different shoulder? (front sight base?)
Secondly, because of the "crush" when tightening the extension onto the barrel, how are you ensuring that the index pin in the barrel extension aligns with the witness mark that you made earlier? Despite the measurements, the exact alignment is still made by turning the the barrel extension, right?
If anyone has drawings for a middy barrel (or at least the critical dimensions) please let me know.
Dave,
Great questions - The length to the shoulder at the breach end is from the front sight base shoulder / step up near the gas port as this is considered a foundation dimension. On an M-4 bbl, this dimension is 6.76".
The length of the barrel tenon is the "threaded shank" that the extension screws on to. This length dimension is CRITICAL as excess length will contact the outside face of the bolt - this will prevent the bolt from rotating and locking with lug alignment.
The extension is screwed on finger tight - the alignment pin and the witness mark are at the required 5-8 degrees out of radial alignment at this point (depending on bbl steel) When I torque the extension on (150ftlbs), this is when I get the shoulder crush - bringing the last 5-8 degrees around will set back or crush the shoulder - this is what necessitates the headspace being cut .002 long - the crush tightens this back to the min 1.464.
Does that clarify a bit?
Cool thread, cool project. Pics and range report would be nice, if your taking tall orders:D. Thanks for sharing.
Thanks opsoff1 - great posts!. "Barrels tenon" and "crush" are terms also used for bolt action rifles and revolvers so we're on the same page. My confusion was with which shoulder you were referring to in your post.
When re-barreling a revolver or a bolt action rifle having sights on the barrel, rarely do the sights end up exactly at 12:00 despite calculations based on thread pitch and crush. The final alignment is usually done by rotating the barrel or receiver sightly until the sights are at Top Dead Center. My question was whether you found this necessary or do you find that the index pin is at 12:00 when properly torqued. If necessary, how do you align it to your reference mark? Mk1 calibrated eyeball or some other method?
If the shoulder-to-shoulder length for a carbine is 6.76" , did you ever find out what the proper length is for a middy gas system?
Dave,
The shoulder at the breach in is faced to allow the extension & corresponding index pin to set up finger tight against the shoulder at 5-8 degrees out from alignment. When torqued to 150 ftlbs, the torque/shoulder crush and thread engagement will bring the pin and witness mark into alignment - personally I use the Mk1 Eyeball for calibration - pretty easy to see and align the witness mark to the center of the pin hole.
When I tighten / torque the extension on - the barrel is still in the lathe as it makes everything waaaay easier if I have to go back and take a light facing cut. I just lock the spindle/chuck with my backgears. (South Bend 10K Tool Rm Model) Doesn't matter if it is at 12:00. As long as I can see the witness mark - good to go.
Does that help?
There is a gentleman who is sending me prints/drawings of Mid & Int bbls - so I don't have those dimensions off the top of my head.
Scott