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Posted: 5/11/2017 7:12:34 PM EDT
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20" LaRue OBR. Over gassed with suppressor (Specwar 7.62).
Has .308 rifle buffer. Has port selector switch, and it is set to suppressed setting as appropriate. Original pre-order OBR so it had the rifle spring and rifle (.308) buffer, now they use the carbine springs/buffers. BACKGROUND Would double feed, and speaking with the guys at LaRue they said over gassed, carrier moving too fast. Tried conversion to carbine buffer/spring with Springco red and orange and H3 buffer, no luck. Converted my buffer to 4 tungsten weights and spacer (7.62 oz) but still double fed. Stock (red spring, carbine buffer) it won't extract/cycle the action with selector on suppressed but no suppressor (so I don't think the suppressed gas is oversized. FDederal XM80 as well as ZQI (both behaved the same) Put in 6 tungsten weights (10.20 oz) and it runs fine suppressed, but unsuppressed it won't lock back on empty mag. Will lock back suppressed. QUESTION I plan to use suppressed all the time. On rare occasion I may use it unsuppressed, will it be a problem not locking back? I never tried to see if it will feed (ie I just put in one round to test for lock back, didn't try loading 2+ rounds). Any problems with "too" heavy of a buffer I need to watch out for? I can try 4 tung and 2 steel (8.59 oz) or 5 tung one steel (9.40 oz) but I'm kinda sick of fooling with it and wasting ammo. ALSO, I'm afraid my double feed may just decrease to a low enough level that I won't notice for a while and it is easier to pull charge handle to fix it not feeding v. trying to clear a double feed (some of them got REALLY jammed up) requiring mortaring and even tapping on charge handle with a mallet). ETA: per FAQ's this would sound more like failure to extract. To be clear, here are some pics: 
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It sounds like when you're running suppressed, that your bolt carrier is unlocking before the chamber pressure has dropped enough to get reliable extraction. If the bolt carrier unlocks before the chamber pressure has dropped, you will often times see a fail to extract (where the empty casing is left in the chamber while the bolt carrier assembly goes rearward). When suppressing with a baffle type suppressor, often times you will need to slow the unlocking of the bolt slightly until the pressure has dropped, and the bolt carrier can now extract an unswelling casing from the chamber.
To slow the unlocking, you can close the gas that flows through an adjustable gas block, or add more resistance to the action going rearward. The add more resistance to slow the unlocking cycle of fire, you can either add a heavier buffer and/or bolt carrier, or a heavier tension action spring (buffer spring). A heavier resistance action spring, will slow things going rearward in the unlockng cycle, but will often times speed up the cycling rate going forward. A heavier buffer and/or bolt carrier assembly, will move slower rearward and slower forward. When we build and tune 308 AR type rifles, I like to use buffers from HeavyBuffers.com. I am not sure what stock system you have, but if running a rifle length stock system then I would suggest staying with a rifle length buffer & spring set up, and not a carbine length. CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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Thanks. All covered in the OP. Rifle length spring/buffer and I have am currently using 10.2 oz buffer.
Asking if I should lower the buffer weight to get it to lock back on empty mag when unsuppressed (but risk having more FTE's suppressed). Also, unless familiar with LaRue OBR's, it has a selectable gas block for suppressed/unsupressed. |
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The PST is two gas ports.
With the block switched over to suppressed, this blocks one of the gas ports, since the suppressor increases the bore back pressure/increases the gas port pressure. https://www.ar15.com/forums/t_2_219/181922_Pg_1_UPDATE_______The_New_LaRue_OBR_Port_Selector_Technology__tm__REVEALED_.html So start with both ports open, and make sure that the rig will run fine without the suppressor. If it will, then the both ports are sized correctly over all. Now install the suppressor, switch the toggle over to suppressed to kill that gas port, and see if the rig will run suppressed. If it will not run suppressed (will not lock the bolt back on the catch after the last round fired), then we need to figure out if the rifle is over gassed, or under gassed. This is easy, and all you have to do it look at the spent case rim. If over gassed (which I bank it is), then the bolt is unlocking too soon with too much residual pressure still in the bore as the spent case is being extracted from the chamber, and the extractor is going to bend the hell out of the spent case rim. If the action is not cycling due to not enough gas port pressure to cycle the action, then the spent rim case should not be bent instead. To really bottom line it, the PST is more twin port sized for Larue suppressor design and the amount of Back pressure it going to increase when in use for the port sizes. With the non- Larue can, it can be creating either more or less back pressure in the barrel bore that Larue's can instead (bank a hell of lot more). So short of running a Larue M308 (what the PST was design for), your back at the needed buffer/spring tension game to get the rifle to run instead. So, run the specwar 7.62 with the PST in suppressed mode, and determine if the rig is either over gasses, or under gassed via the spent rim to begin with. With any luck, will find that you are only slightly over or under gassed, and may be able to run just a slightly lighter (undergassed) or heaver (over gassed) buffer to not only solve the can problem, but still allow the rig to run un-supressed with the PST set wide open as well. If way off the mark instead regarding the amount of gas, them may end up play the two different buffer game to run the rig suppressed and unsupressed instead. If you need help reading the spent cases, then post photo's of them. Also, post photos of the spent cases of suppressed with the PST set to suppressed, not set to suppressed, and then no suppressor with the gas system set to none suppressed. Hence need this none suppressed case as a base line, for a final adjustment to specwar 7.62 supressed with the PST set to supressed as well. FYI, The bottom one is your can, and with the limited blast section space before the first baffle and your adapter design, it has a shit load of back pressure. The M308 design (not the top suppressor) lowers this blow back pressure instead (LEPE). 
[youtube]https://youtu.be/jQH8223J89E[/youtube] |
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If you really want to slow the carrier down and slow the unlock time put a extra power hammer spring in the gun as an easy fix unless you are running a special trigger. The way the gun works most of the brunt of recoil is taken up in the camming downward of the trigger.
Though adding more mass to the recoil system does solve problems more easily as it has to move more mass backwards, the trigger spring can and will eat up a LOT of it. Then again im still trying to figure out why someone built a gun with a gas system opened up as much as you are saying provided all the setting are in check |
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Thanks for replies. I thought OP was clear. It IS over gassed. I've already put in the heaviest buffer I can (as well as several intermiediate weights) and it works fine suppressed (with selector on suppressed setting), but unsuppressed (with selector on unsuppressed setting) it will not lock back on empty mag.
Is the not locking back on empty mag likely to be a problem (short stroking/not picking up a round from a full mag)? I can lighten the buffer a little without getting to the last weight that caused problems (suppressed) but don't want to waste a bunch more time and ammo. I'm likely to leave as-is since it will almost always be suppressed and clearing the double feeds is MUCH harder than just having to pull the charging handle to load a round..... |
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Quoted:
Then again im still trying to figure out why someone built a gun with a gas system opened up as much as you are saying provided all the setting are in check |
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