Accuracy improvement with different gas block

[ss355]

Prior to my range run today, my "Grendel" rifle (.264 LBC tube) shot what I considered lackluster groups--certainly not the ragged, one-hole groups that I've Satern owners post in pics here on 65G. I spent north of six Benjamins on the barrel/bolt combo straight from Les Baer, so I was disappointed that the barrel printed an average of around 1 to 1.25 MOA with factory ammo.

Before I built the gun I read (perhaps on 6mmbr.com?) that for a target gun one should opt for a clamp-on gas block rather than a set-screw block because the set screws could deform the bore. I wanted to get a clamp-on block for my .936" gas block seat, but the only one I found in steel, my preferred material, was a Model 1 Sales block. I turned up my nose at the M1S block, figuring it was inferior, and opted for a steel, low-profile block that employed set screws that I got from either Rainer or Palmetto. After all, how could relatively light pressure from a couple tiny set screws deform a heavy barrel wall?

I don't know if the set screws deformed the barrel wall, but it seems the clamp-on gas block that replaced the old set-screw block has made for an improvement in accuracy. Perhaps the much larger, heavier clamp-on block changed the barrel harmonics, but whatever the reason, factory Hornady 123 grn Amax is printing groups about half the size of groups I've shot in the past. I still need to shoot more groups in duplicate conditions to confirm, but so far my results are encouraging. Oh, and I used blue loctite under the clamp-on block during installation, whereas the old set-screw block went on sans loctite. I believe the new block is the M1S block I eschewed a year ago. I picked it up on the cheap from the ARFcom equipment exchange.

I'm curious if anyone else has experienced similar results.

[Whelenon]

Good question, I've got the set screw gas block and wondered the same thing. Looking forward to a response from someone in the know.

[DHumbarger]

Maybe the steel blocks being heavier than the aluminum ones interupt/disrupt the natural barrel harmonics thereby adversely affecting accuracy. I can envision that a gas block located farther out on the barrel might tend to have a greater affect than say a carbine length block. Might be that on a carbine length block steel is ok & on a rifle length block aluminum is the correct one. Just food for thought.

[bwaites]

Most accuracy gurus have completely discared aluminum blocks because the thermal expansion properties are so different from barrel steel, and they feel that it complicates issues once the barrel heats up.

Most guys building Grendels have been bedding the gas blocks with loctite for some time, I believe. There is a thread somewhere on here where Variable talks about it.

[ss355]

Bill, I too have read the same thing about thermal expansion coefficient differences between steel and aluminum, so that's why I wanted to stick with a steel gas block.

The old gas block was a steel, short, low-profile job that was very small and took up only 2/3 or so of the gas block journal on the barrel. Even though it was steel, because it was so thin it weighed next to nothing. The new block is also steel, much heavier with a picatinny rail on top, and spans the entire length of the gas block journal. Could be that the weight alone of the new block has an effect on barrel harmonics, or perhaps the weight combined with the clamping action is what's making the difference. Regardless, I'm pleased with the results thus far.

I also have a DPMS in 6.5 Creedmoor that has an aluminum, set-screw gas block that came with the gun, and it shoots so-so groups with factory ammo. I'll get another M1S steel hvy bbl gas block to try on it to see if makes any difference in accuracy.

[stokesrj]

I wouldn't claim to know all there is to be known about gas blocks but I have some experience and some observations. I think I've built around 20 uppers or so now, about half in 6.5 and the other half various calibers mostly .223. l've used both steel and aluminum, set screw, clamp on, bedded and not. I haven't seen much difference in any of the combinations. However, I've seen some negative impacts on accuracy in how the gas tube is set up.
The gas block must be in a vertical position that centers the gas tube in a torque free path through the barrel nut and receiver key way. If this is not done it will impart variable harmonic influence on the barrel. The proper way to do this is to mount the gas block dry, insert the tube in the gas block tenon but without the roll pin installed and sweep it back and forth above the action. Observe the swing to either side of the center line of the action and adjust the gas block so there is equal swing on either side.
By correcting this gas tube alignment I've turned 1.5 MOA guns into .5 MOA guns.
Bob

[DHumbarger]

Good info on the gas blocks. Insights like this is what makes this site so valuable.

[Variable]

Most guys building Grendels have been bedding the gas blocks with loctite for some time, I believe. There is a thread somewhere on here where Variable talks about it.

Yep. http://www.65grendel.com/forum/showt...ical#post28878

[ss355]

... I also have a DPMS in 6.5 Creedmoor that has an aluminum, set-screw gas block that came with the gun, and it shoots so-so groups with factory ammo. I'll get another M1S steel hvy bbl gas block to try on it to see if makes any difference in accuracy.

Swapped out the aluminum, set-screw gas block for the steel, M1S clamp on and got the Creedmoor to the range today. Unfortunately, I didn't detect any improvement in accuracy with the new gas block with either of the factory loads I tried. Oh well.

[ricsmall]

355, I would probably check the torque on the barrel nut and the amount of slop b/w receiver and barrel extension, and the gas tube alignment procedure that stokes mentioned. I have seen people "bed" the barrel to the upper receiver with blue loctite to enhance accuracy. Good luck and keep us posted.

[ss355]

I might give that a try, ric. Another DPMS factory rifle I had, an LR-308, had the barrel nut torqued on so tight I had to use heat in addition to my heavy duty strap wrench when I removed it to sandblast it for a matte finish. However, I don't recall the barrel extension being a tight fit in the upper, so could very well be that this LR-6.5 could benefit from the blue loctite treatment.

Oh, and Bob, a question regarding your gas tube centering method. Does centering the tube using this method typically leave the gas block at top dead center? I know the block vent hole is large enough to accomodate some some side-to-side slop and still handle the full gas volume, but I was curious if the block had a rail if it typically aligned with the rail in the upper. I suppose it's a moot point for me because I'm running glass only on this gun with no back up sights.