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.308 XL Length Gas Port
- Thread starter bigjake83
- Start date
I'd be very interested too. Should work well with some of the stouter loads using 175-185 grain bullets. If anyone shares please also tell us your barrels length.
If you'll forgive my ignorance, what's a +2 gas system? I've seen reference to it in multiple places, but can't find how it compares to carbine, mid, or rifle length tubes.
Its a gas system that is 2" longer than rifle length gas. It has almost become the norm for 6.5cm and 260rem AR10 barrels these days. You can fund most of the top barrel makers and smiths offering +2 gas systems now on the above mentioned barrels. I havent seen it on 308 barrels and not sure its necessary but Im not sure...Only one way to find out. Have a barrel spun up with +2 gas in 308 and test it out.
Makes sense, thanks. When I built my 308 AR a couple years ago, I wasn't even aware that was a thing.
I'd definitely be interested in this too for a 308. I'm not sure, but was it JP that was the one who first started offering the +2" gas length on his 6.5/260 rifles...? Still drooling over his gas guns! One day damn it!!!
Yes it was JP who commercialized it first, but other small gun smith had already been using it, but that does beg the question if it is an improvement why hasn't JP started running their .308 barrels with the + 2??? So maybe there is a issue in hiding we don't know about yet with the +2 on a .308
Just a guess but the 308 may become very picky about powder selection to get it to cycle with a +2 gas system.
I'm basing that very loose guess on my experience with a smaller 30 cal wildcat in the AR15. My wildcat only uses 30-35 gr of powder and I found with a rifle length gas system and 20" barrel, even with a .125" gas port it only cycles correctly with a few powders; others that give good ballistics and accuracy just don't produce enough gas to cycle, obviously I should have used a mid-length system for that one. The 308 certainly uses more powder and therefore produces more gas, but if the gas port is +2 length, it might be close to having the same problem as my wildcat. Then again, if it's only used on long barrels, maybe it's fine.
Have you guys had trouble with early bolt unlocking on rifle length 308 systems when the gas is adjusted properly? If so, what barrel length? My experience is only with a few samples, all DPMS pattern and 18-20" barrels; when the gas is adjusted properly they all run correctly even with max loads. One of those (my personal 308 right now) is even with a lightweight bolt carrier, and no issues except that when gassed for the hot loads they won't quite cycle 7.62 mil surp.
I'm basing that very loose guess on my experience with a smaller 30 cal wildcat in the AR15. My wildcat only uses 30-35 gr of powder and I found with a rifle length gas system and 20" barrel, even with a .125" gas port it only cycles correctly with a few powders; others that give good ballistics and accuracy just don't produce enough gas to cycle, obviously I should have used a mid-length system for that one. The 308 certainly uses more powder and therefore produces more gas, but if the gas port is +2 length, it might be close to having the same problem as my wildcat. Then again, if it's only used on long barrels, maybe it's fine.
Have you guys had trouble with early bolt unlocking on rifle length 308 systems when the gas is adjusted properly? If so, what barrel length? My experience is only with a few samples, all DPMS pattern and 18-20" barrels; when the gas is adjusted properly they all run correctly even with max loads. One of those (my personal 308 right now) is even with a lightweight bolt carrier, and no issues except that when gassed for the hot loads they won't quite cycle 7.62 mil surp.
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[h=1]JPSM308-22M11R[/h]
| Manufactured to our exacting specifications and used in all of our rifles, JP Supermatch™ barrels are turned from 416R stainless steel, lead lapped, air-gauged and cryogenically treated for cold bore shot predictability, minimum thermal drift, outstanding accuracy and extended barrel life. This barrel also includes a JP EnhancedBolt™ individually head-spaced to the barrel for flawless fit and function. Caliber: .308 Win. Length: 22" Gas System: Rifle-length <<<<<<< really?? Twist: 1:11.25 button rifled Contour: .750 under hand guard/.875 in front of gas block Diameter at Gas Block: .936 Muzzle Threads: 5/8"-24 TPI Material: 416R Stainless Bolt Assembly: JP EnhancedBolt™ Availability Notice |
I run both .260 AI and a .308 AR's with +2 gas tubes and SLR adj gas blocks on 22in barrels. Run both suppressed and both have the JP VMOS BCG and High Pressure bolts. I found that even with the gas turned down the bolt was unlocking early. Solved with the VMOS.
Does the plus 2 on the 308 smooth that out a bit, not so harsh on the brass??
It does but the biggest help was the VMOS and heaviest SCS. I started with the LMOS hoping that I could regulate things with the gas block...no go. Then JP came out with the VMOS. Not unlocking early is the big thing with the brass. I don't run hot loads but the back pressure from the TBAC Ultra 7 was overwhelming the gas block. I'm running AA or IMR 4064 currently, but plan on running up the scale to Varget and down to 4350 to see if that makes a difference.
Correct. Before that I shot with a Holland muzzle brake. Now that's where the 2+ gas system shined. I could watch the hole appear in the target! Although most everything I've read about shooting suppressed says it's smoother and less recoil, I don't find that to be true. The Holland muzzle brake is amazing though.
Well, even though I live in a rural area, the neighbors appreciate it because I shoot alot. And, now that I have the bugs worked out it's really no trouble. No change in accuracy. It's all good.
This part doesn't make sense to me. Would your gas block not close down far enough, or did you have it tuned without the suppressor? Tuning a decent adjustable gas block with the suppressor in place should make the system work correctly (always has for me anyway); there is no "overwhelming the gas block" unless you just ran out of adjustment.
I use my 308 AR suppressed with a light weight carrier that is lighter than JP's LMOS and have no issues, even with very hot loads. But my gas blocks will close down far enough to shut off the gas completely, so the adjustment range is infinite.
I have owned a +2 308 AR for many years. I bought the barrel from ADCO. They offered a 24 inch +2 Pac Nor barrel for the AR10B models when Armalite was the only game in town. I also recall that Hollinger (WOA) experimented with extended gas systems for the AR platform. This was10+ years ago and I believe he even built one with a +3 gas system. But this was back in the day when it was more common to have 24-26 inch barrels on ARs esp. varmint/target models. My guess is that the +2 gas movement kind of died on the vine as the trend towards shorter barrels took hold. Just for reference my 308 upper has a SWS 15" rail installed to cover the +2 gas system.
Edited to add: Mike in FL I am impressed that you have that link. I lost that link years ago. Hollinger's +3 gas system is one of the rifles pictured.
http://www.radomski.us/njhp/ar10/jh_carbon/rifle.htm
Edited to add: Mike in FL I am impressed that you have that link. I lost that link years ago. Hollinger's +3 gas system is one of the rifles pictured.
http://www.radomski.us/njhp/ar10/jh_carbon/rifle.htm
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http://www.radomski.us/njhp/ar10.htm. there's some 15 year old ext. gas .308s.
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Yondering,
So...I thought it strange too that my gas block wouldn't control the gas enough. I have an SLR gas block and tuned it with and without the suppressor. I could turn the gas down but the bolt wouldn't reliably lock back so I tuned it so the bolt would lock back reliably, but...the bolt was still unlocking early. The brass was sooty, initially around the shoulder but as I shot more it progressed down the case. Once I had the VMOS in place that stopped...bright shiny brass, no soot.
I'm also going to have to check one more thing...the body measurement of the fired brass. My theory is that with the bolt unlocking early, the brass bodies will be larger...just like a Garand or M1A. I'm having the gunsmith make a gauge using the chamber reamer. I'm finding out that, with these rifles, there's more to gas system adjustment than the bolt locking back reliably.
So...I thought it strange too that my gas block wouldn't control the gas enough. I have an SLR gas block and tuned it with and without the suppressor. I could turn the gas down but the bolt wouldn't reliably lock back so I tuned it so the bolt would lock back reliably, but...the bolt was still unlocking early. The brass was sooty, initially around the shoulder but as I shot more it progressed down the case. Once I had the VMOS in place that stopped...bright shiny brass, no soot.
I'm also going to have to check one more thing...the body measurement of the fired brass. My theory is that with the bolt unlocking early, the brass bodies will be larger...just like a Garand or M1A. I'm having the gunsmith make a gauge using the chamber reamer. I'm finding out that, with these rifles, there's more to gas system adjustment than the bolt locking back reliably.
As far as who did it first, David Tubb used extended gas tubes in the early to mid nineties (pre tubb 2000)in his two Sr-25 6.5-08 (pre 260 remington)match rifles, thats my earliest recollection of someone using long tubes ....as far as the OP question it will work, start small on your gas port and work up....
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