Albroswift

Before my incredibly short attention span and Al-zimers moves me along to the next project, I'm posting up some musings on blowback buffers. Not claiming to be an expert in the subject and by no means a complete dissertation. Any-hoo: Tried several different combinations of springs and buffers over 3 AR40's, 2 with 4" barrels and 1 with a 16" barrel. First of all, 3-1/4" buffers don't quite work. End up with too much travel and with LRBHO the bolt is moving way too fast when it hits. First solution was a spacer to limit travel. Spikes sells one as well. Works OK, but space could be put to better use. KVP has a 4-1/8" 7.5 oz solid SS or 10 oz Stainless with 4 tungsten weights. The solid didn't work well for us, too much bounce, my opinion the weights really help deaden the effect. The 10 oz (Mine weighed 10.29 oz) worked good and fair priced. Problem with the KVP is it is designed for a spring that compresses to about 3-1/2" which as far as I know doesn't exist. Then tried the Quarter Circle 10, 11.2 oz, tungsten weights, and the head is a little bigger so about the correct compression for a Sprinco Red or EA1095 spring. A little shorter overall then the KVP's and made out of material that rusts! Heavier then SS? Ran really, really smooth in the 16" AR40/ red spring. Two problems with the QC10, damn thing is 120 bucks, and when I was going to buy another one anyway, found out they quit selling them! QC10 now only sells firearms it looks like. So, shop time. First bored the KVP 7.5 solid to hold 4 weights, came in even heaver then the QC10 or the KVP 10, possibly due to the bore was a little tighter. Added a collar to set spring compression. Also turned a monster that holds 5 weights, set up for a red or EA spring in a 7-11/16" extension tube. Some deductions: If you are already running a KVP 10oz and a Sprinco orange, a SS collar can add some weight, 1.5 oz, +/- and sets the orange spring for full compression. (Sprinco Orange was designed around a 2-1/2" buffer in a 7" tube and 308 BCG I believe) Sprinco Red and the KVP, a 5/8" 0 .75 oz spacer sets the spring compression and brings the total up to 11+ oz Some more picks: Some good information here: The American PDW https://10mmautocombat.wordpress.com/blowback-bolt-calculations/

Holy moly, found 2 sticks of 303 stainless on ebay, bought it, delivered today, they sent me all they had! Not only that, invoice is from KVP! Got plenty of KAW VALLEY buffer stock now, that's for sure

KVP 4-1/8" 11.5 oz Their SS is not quite as mill friendly as the SS I used on the 4-13/16" buffer. Got er though.

The problem I have with the KAW is it's a solid, offers no dead blow effect. I think that is as huge a factor as is weight. Almost done boring out my KAW SS 7.5 oz for 4 weights, should match performance the discontinued QC-10 that I have great luck with in the AR40 carbine, maxes out at 11.3 oz with 4 tungsten. For the 9MM maybe 2 tung and 2 steel, +/-

That plus a 19oz bolt seems about right little under 2.2 lbs. I've been soaking in an article by a fellow named Chinn, he calculates it at around 2.8 lbs. Couple excerpts: "The 10mm level of power really represents the upper level of the blowback action due to the necessary weight of the bolt. Any more power and the bolt becomes so massive that the weapon becomes cumbersome, the exact opposite direction we want to go. If we add a locking mechanism, the complexity of design goes up, again not the direction we want to go. The 10mm shows its excellence by delivering the most force out the barrel controlled by a bolt weight within a reasonable consideration." “NOTE: There is one point which requires special clarification at this time. In many descriptions of blowback actions, it is strongly implied that the driving spring contributes a substantial portion of the resistance which limits acceleration imparted to the bolt by the powder gases. Actually, this is not so. Although it is true that the driving spring absorbs the kinetic energy of the recoiling bolt and thus limits the total distance it moves, the resistance of the spring does not have any real effect in the early phase of the cycle of operation. The bolt acceleration occurs mainly while the powder gas pressures are high and are exerting a force of many thousands of pounds on the bolt. The driving spring, in order to permit the bolt to open enough to allow feeding, must offer a relatively low resistance. Although this resistance is sufficient to absorb the bolt energy over the comparatively great distance through which the bolt moves in recoil, it is not great enough to offer significant opposition to the powder gas pressure until the chamber pressure has dropped to a relatively low level well after the projectile has left the muzzle.” Good article, The American PDW https://10mmautocombat.wordpress.com/blowback-bolt-calculations/

Yea baby....

Actually wasn't Odin, theirs only hold 3 and are 3-1/2" long, even for their PCC buffer. It was a Quarter Circle 10, found out when I tried ordering another one. Odins are all 3.5, QC10 are discontinued. So making a couple. First one is a KVP 4-1/8" Solid SS that I'm boring out, adding a collar for correct spring compression. Should match the QC10's 11.3 oz. Second one on the lathe right now is going to be a 4-13/16" 5 weight model for 7-11/16" AR10 tubes. Still keeping the tail at correct spring compression dimension. Waiting on a long end mill to ream and flatten the bottom of the bore. Adjustable weight on that one should be close to 15oz

One less now! Thanks for the link.

I've had the charging handle hit the end plate that just needed a few strokes of a flat file to fix. Worked fine in the open position but hung up when closed, checked about a million things before I got to the DOH! moment.

Range day with the labradar and the 10mm The AAC#9 cycled the gun and was hitting 1475 fps at not quite max load, good sd's for a pistol. No primer flow. The 6" ported barrel made the gun comfortable to shoot even with the hot loads. For a baseline the factory Hornady 155 xtp loads clocked at 1275 fps . The VV3N37 was disappointing, the low end of the ladder wouldn't eject, acted like a single shot. Even getting close to max barely clearing the ejection port. Found some XTP specific data a little hotter, loaded 15 of those with the 3N37 and loaded 30 more of the #9 at .3 gr more (max load) should hit 1500fps! Like to get the 3n37 to work, have 6 or 7 lbs of that, only 1 of the #9.

I'm sure you can do it with the dies, sounds like some extra work, and yes I would FL size each time reloading in any autoloader. I have a small base die for the 308, only use it on gravelpit ammo, don't ever really seem to need one on any other calibers. Your results may vary.

I kinda thought you might be exaggerating just a bit, but I wasn't sure because you left out the media blaster step between the brake cleaner (Non chlorinated of course) and the hot soapy water.

Holy moly, do you do that when changing powders in your powder measures?

Hate sorting brass with the press. Have a bag of the small primer 45's, not enough for a run. The 10mm, though, I have 200 small and 50 or 60 large so running with the small for now. The 10 seems a lot easier to spot the difference then the 45.

I used RL-15 for 25 years but am getting better results with their newer AR-Comp in the 308ARs. Still use the 15 in lever guns and my H&R SB Ultra. Data shows lower pressures required for similar velocities so seems to me a good long barrel powder, but maybe too much gas for my 308AR's midlength and carbine setups

Bad eyes, Blazer not Starline. Bid on several batches of brass on GB. Guess that's what I got! (One with the hole I drilled in the lathe to check barrel alignment)

So started out this morning, working up a couple ladders for the 10mm. Brand new starline brass, noticed it has small pistol primers, and all the data is large pistol primers. Actually may be better, the factory loads were not too kind on the large primers, looking like I was getting a bit of flow. Loaded 2 ladders, one with AA#9, and one with VV3N37. The VV I have used in .40 and quite a bit in 9MM and 9MM Major, always had good burn so used Win small pistol primers. The AA#9 only place I've used it before was magnum loads, went with Federal small pistol mag primers. Hopefully run them across the chrono next weekend. Don't have a lot of other pistol powders on the shelf to try, some tightgroup and H110, probably off the fast/ slow ends of the spectrum.

While you can make a 7 inch tube work, no way is there a combination out there that a 3.5 oz buffer will work in a 308. Edit we'll get some more specific help to you, but first place to start is 5.4 oz buffer and a ea1095 spring in a 7-11/16" tube.

Spikes Tactical 14.5" 5.56 1:7 5/8" journal midlength gas .078 port diameter

Just thinking out loud, If I were building a blowback 10mm, I would go with the AR 10 carbine tube, and turn a 4-3/4" buffer that would take up to 6 tungsten weights. My guess is you could get up to a little over 14 oz total buffer weight. Could also make some different "collars" / spring spacers to change the length of the head to tune the spring rate.

You could use a 7-11/16 tube and the 4-1/8" Odin

I think the sliding weights and pads were the last piece of the puzzle. The red spring is for longer buffers could be the spring rate of the longer spring helps as well. The orange spring doesn't get compressed with the longer buffers. It's a thought anyway. Really like the .40 for a PCC caliber but they seem to be a tricky bugger to get balanced.

Back out with the 40 pcc. Replaced the 10oz solid buffer with the Odin, 4 tungsten weights, Sprinco red. 4-1/8" buffer came in at 11.36 oz Weights and pads fit tight, so sanded a little bit off the bumper to get a little bounceout of the weights. Put Wilson 33 round 9mm springs in the old 22 round .40 mags. Speed up the feed slowed down the bolt. Was running reliability but still getting the occasional double / triple burst, this cured it, no double taps and 100% feeding 155 gr XTPs in the 1500 fps range. Going to watch for a deal on a couple more Odins, do my pistol and the brother's while I'm on a roll.

First thing I thought of.

That's as good as it gets brother. Gotten down in the 3-4 range before but don't even get a 1 in a 1 shot string.