Hey guys:  I need some help so here is my MEGA Porn contribution.
Just finished my Mega assembly in 260 Remington.
Specs:
Mega Upper and Lower
Timney Trigger
Misc. Upper and lower parts
SLR 15” Tube
Magpul stock
Hogue Grip
Cryptic Coatings Bolt carrier assembly in Mystic Bronze….black was not available at the time of order…kind of growing on me now.
Barrel built from Hart Blank by Teton River Traders….fine Job. ….24” HEAVY RIFLE LENGTH GAS
SLR Adjustable Gas Block
Carbine tube
Standard carbine buffer and springs…Buffer length 2.52”…….spring 28 coils 11.25” long wire OD 0.072”
Warren Rings and Weaver 36 power mounted for load development

PART II

I love the gun.  It shoots great and seems to have some serious accuracy potential.  I have done a little load development with 130 bergers, 140 SMK and already recorded a few 5 shot groups in the .4’s.  I see our relationship turning in to a long one………maybe.

From the very beginning I could not get the bolt to lock back on the last round and lots of times the bolt carrier will not seat all the way forward……..I have to nudge it into place to completely seat the carrier before I can shoot the next round.  It always strips the next round from the mag and always successfully ejects the spent case.
•Yes there is enough room for the bolt to travel far enough back…. I would say about ¼” or so past the bolt hold open.
•Yes I have adjusted the gas block until I am getting some hard recoil from the buffer bottoming out so I know it is traveling all the way rearward…..I can adjust the gas block closed until it short strokes and then work my way back up to strong felt recoil and still no bolt hold open and lots of failure to seat fully on live round.
I am working with H4895 for load development.  It appears to make no difference if the loads are light or very hot I am still having the same two problems.
•Bolt will not stay open on last round fired
•Bolt fails to seat fully forward on live round most of the time.
Bolt and bolt carrier appear to fit well.  Bolt was headspaced to my barrel and fitment is great.  I can slowly ride the bolt forward and it will close completely.  I have compared headspace length between fired and loaded brass to make sure they were a few thousands shorter and they are.

PART III

I am to the point that I assume I should get a stronger recoil spring and maybe a heavy buffer????
So…………what say ye experts.  I have only been messing with AR’s for a few years most of my 30 years of experience is with Remington 700’s.
Thanks in advance for all the help.
Eddie.

Im not an expert...........but, I would try factory ammo......and see what happens.........any time I complete a build, I always test fire with factory rounds..........Then move on to reloads............

As for the heavy spring, I put one on a rifle that was choking and it made things worse.........I eventually had to drill out the gas port on the barrel...........

I would get a JP SCS.  Your recoil spring-is it an LR308 carbine spring?

Try lubrication, using Slip2000 if you want to keep messing with the current set-up.  It sounds like you don't have enough carrier velocity in the return stroke.

Try Lancer mags as well.

I have shot some factory HSM ammo loaded with 123 scenars and some factory Black Hills with 175smk and got the same results.

I was thinking that possibly the standard buffer and the buffer spring I have is simply too light for this setup and possibly allowing too high of a bolt speed…..hence the bolt passes back and forth over the hold open latch before it can fully lift???

And maybe the spring is too light to fully seat the bolt forward consistently?  That is my shot in the dark at the moment.

The Smith told me he was going “go up one size on the port diameter” since I was utilizing an adjustable gas block.  I did not measure the exact diameter but I can. Disassembly is fairly painless with this setup.

In the gun is a standard 308 buffer and spring from palmetto state armory see specs above…..no markings on the buffer and I currently don’t have a working scale in that range to see what it weighs.

It is lubed well with slip 2000.   Feels like its running on bearings honestly with that coating on the carrier.

I have tried the DPMS 20rd steel and Magpul 20rd.  They both feed fine.  Everything functions fine as far as holding the bolt open property when you work the bolt by hand on an empty magazine……just won’t hold the bolt open on a firing cycle even when I add gas to make certain I am getting full bolt travel…you can feel the added recoil impulse from the buffer rebounding off the back of the tube.

I like the JP silent capture system but I think I will order an extra heavy spring first unless anyone has more insight on the problem.

Just seems like not being able to correct the problem with the gas block it has to be spring/buffer related.  

This or the Armalite operating spring. The 308 needs a nice balance of spring tension and compression. It's real easy to exceed both ends of the spectrum.

The JP silent spring system is very good, a bit over kill if you're wanting something to just work. The JP system can be adjusted as well i believe.

The armalite spring is designed very well. Many problems can be solved using their buffer system. I chose the armalite over JP because of simplicity. JP is great for a race gun, or competing. The armalite just plain works.

Ive chased my tail on many builds over the last twenty five years, I had a 6x45 build that was doing the same thing you are describing..........my rifle would shoot fine but would not lock back on the last round.......emptied the magazine everytime..........in retrospect I believe a lighter spring would have solved my problems...........but I went with a heavier one.........the heavier spring did not solve my problems............I ended up opening up the gas port and that solved everything.......the fact that you have an adjustable block and I assume you have set it to the point where it is giving the rifle maximum pressure has me thinking its a gas problem........

These gas guns are supposed to work with the basic stuff.........E.G. buffers and springs........I think a lot of it is psychological ..........Ive changed springs and buffers on rifles and they still eject in the same spot........but, you can ask three people what they use and you will get three different answers.......you got plenty of folks say you need an adjustable gas block and others say its not needed.........

that said, I would focus on this being a gas issue, assuming you have the correct buffer and spring in your rifle.....I have a Palmetto State Armory PA 10 and I have noticed the buffer tube is longer than a standard Carbine tube.......The buffer itself is peculiar to the tube and I assume the spring might be as well. Measure your tube and see what the length of it is.......

Good idea. I will measure the depth of the tube as installed. I ordered a "orange spring" from springco to see what happens. I assumed maybe wrongfully that it could not be a light spring issue but maybe so.

Tomorrow I will disassemble and measure the gas port diameter.

Anyone have a set of recommended specs for tube depth, gas port diameter, spring and buffer for what I have put together?

Thanks

Eddie.

The buffer length you posted sounds correct for a standard carbine tube...........

UPDATE.
I am correcting my post above.  Barrel length is 24”…..brain fart.
I ordered the “orange spring” Thursday night and received it today.  I slapped that puppy in there and put 10 rounds I had loaded in my trusty 20 RD DPMS steel mag and fired them all without incident.  Bolt closed fully on every round and actually ended up putting them all in about 1.1” at 100yds.  Not too shabby.  That by the way was using WIN LR primes / 40.0 H4895 /95 V-MAX / LAPUA BRASS  / MAKING 3188 FPS with a SD of 19 fps. Brass was not beat up and showed no pressure signs.  Brass was piling up nicely somewhere between 3 and 4 o’clock.
Again, the bolt did not stay open upon firing the last round so I studied and investigated that a little further.  It does hold open properly when cycled by hand…………. I can manipulate the paddle by hand with an empty mag inserted and it sure appears to lift pretty damn slow.  I believe the combination of the bolt release spring and those mags may be the problem….I had tried MAGPUL 20’s before and got the same results.  I will have to try those again with this new spring.  If that is the only issue I have to deal with I’m not really disappointed.  It’s not like this is one of my home defense weapons and I doubt I’ll end up in Iraq sniping from a roof top or any other combat situation.
I may even still try a X-Heavy buffer to see what that does.
Current gas setting with this set up on the SLR block is 5 clicks out from closed.  I like the block……VERY nice.  I coated the set screw with pure nickel never seez and have had zero issues with it sticking when trying to adjust with 188 rounds thru the gun so far.  Of course I have been playing with it quite a bit during load development so I can’t testify how stiff it may get after shooting 3 twenty round strings thru it with no adjustments but I’m very pleased at this point.  I will update you guys after more use.

Did you look at your buffer,to see if it is hitting your mounting screw? If,you mistakenly use to long of a screw it throw's all of the timing off.I know,it happened to me.
If,this is your problem(hopefully)it is a simple fix.Either,change the screw or cut that one down about a 1/4".

maybe go to a fixed stock?
Or remove two coils on the spring?

It's your magazine.  Lancer's will fix it.  Have a GAP-10, been there-done that.  I must have every magazine ever made for an AR Variant...Lancers, only Lancers. Read DPMS Gen II Quibbles thread under  AR-Variants.  Also, under Trouble Shooting read Why I don't use Plastic Magazines. Check the parts listed in these threads.

UPDATE 3-25-2015

Ok, this evening I shot 3 different loads out of one of my magpul 20 rd mags with the new orange buffer spring installed and for the first time in history it not only held open on the last round but did it 9 times in a row with three different loads……  Amazing.  While I was cleaning her up this evening I noticed a good dimple forming on the bottom front edge of the outer bolt lug (bottom lug)…… so I checked the barrel extension……….. all good and slick no corresponding marks.   Then I looked at the top edge of the bolt catch…..what do you know…… there is a corresponding  bit of damage on the top edge where the bolt has been barely catching it on the way back into battery.
Comparing the DPMS mag and the Magpul I feel noticeably more spring pressure on the magpul….hence it would make sense that it will lift the bolt catch faster…and apparently fast enough.
So now I am thinking the extra heavy orange buffer must have slowed down the bolt speed enough that now my stronger sprung mapgul mags are lifting the bolt catch fast enough.  Kind of makes me want to try an extra heavy buffer.  Does anyone besides slash make a extra heavy buffer for the carbine setup I have?  The Slash is the only one I have looked at.

On a side note, I have been comparing groups with this new gun shot off both my heavy adjustable front rest and a quality Sinclair bipod.  It appears that the groups do tighten up shooting off my heavy front rest pushed up close to the receiver compared to shooting off my bipod at the end of my 15” SLR tube.  While I do love the look and the feel of the SLR tube it is not the most ridged I have played with…….. of course in all fairness it is the longest I have too. The gas block does clear the tube top and bottom sitting on the bi-pod but I wonder if I am getting any contact during a firing sequence?  What do you guys think would be a minimum clearance under static conditions? I have not tried to measure the clearance but it is pretty close.

Sharpnail what mounting screw are you referring to?  The buffer retainer?
Tiger222 I think my problem was too little buffer spring pressure.  It would not close the bolt on a live round reliably.
gr8 I will try to pick up a Lancer mag.   The magpul’s I have certainly have a stronger spring than the DPMS.  I have had zero problems with either the DPMS or the Magpul in my SR7.62 over the last year and about 500rds.

Please disregard my ignorant post,it won't happen again.Apparently,my head was elsewhere.

Update for you guys.  

Zero malfunctions in the last 200 rounds.  Rifle is doing great.  Especially with the lighter bullets…….  95 vmax at about 3500 are grouping great.  5 shots into .5 moa on a regular basis.

Lapua 123’s in the .4’s and .3’s at about 3000 fps.  Any faster and the groups open up and do not tighten back up before I start piercing primers.

Love the SLR adjustable gas block. I can tune the rifle to minimal recoil and excellent reliability.  However, if I go more than 20 rounds or so without cycling the adjustment screw it starts to get pretty hard to turn.  I have pure nickel never seize on the threads but it still gets bound up with carbon fowling.
Anyone have any better luck with that gas block?