Anyone use a COLT Sporter or Match Lower with a Grendel

[vaguru]

I have a 20" bbl upper and have tried in on a Colt Match H-Bar lower with Armalite 2 stage match trigger. It lacks the ability to feed rounds from magazine or lock back on the bolt. It does lock back but catches on the bolt carrier.

The bolt carrier that is in the upper is a full auto carrier whereas the original Colt carrier has the bottom rear of the carrier removed. Buffer is 5 oz. and buffer spring is correct length and coils for A2 mil spec stock.

Gas port in barrel is .096" and aligned properly. I have run out of ideas on what to check next.

I only have 2 lowers, both Colt ( one Sporter and the above Match), both originally H-Bars, and both have 2 stage match triggers, the other is a Kreiger/Milazzo version.

Thanks for any thoughts.

[montana]

Explain won't feed rounds from magazine. Are you able to chamber a round and eject it by hand with out a problem? What type of barrel and bolt do you have? Does your upper fit the lower OK? Does the barrel feed ramps look OK in the upper? What type of magazine are you using? Is the gas tube old-new-fit the gas key properly? The Colt carrier was an older anti full auto type to prevent the rifle from tripping an auto sear.. Did you try using both lowers on your Grendel upper? Need more info..

[vaguru]

Explain won't feed rounds from magazine.

Bolt carrier does not go back far enough to pick up rounds from magazine or to engage bolt stop. Engages on bolt carrier front rather than bolt. Pulling charging handle back allows bolt to pickup fresh round from magazine.


Are you able to chamber a round and eject it by hand with out a problem?

Can chamber and eject rounds by hand using charging handle fine. Rifle fires and ejects spent case.

What type of barrel and bolt do you have?

This was a complete upper from well known company. Bore scoped barrel before shooting, looks very good throughout.


Does your upper fit the lower OK?

Upper fits lower perfectly, no binding anywhere.


Does the barrel feed ramps look OK in the upper?

Feed ramps not an issue, feeds perfectly using charging handle.


What type of magazine are you using?

Used both AA Elander and Duramags, no difference.


Is the gas tube old-new-fit the gas key properly?

Complete new upper and parts, tube not rubbing on key.


The Colt carrier was an older anti full auto type to prevent the rifle from tripping an auto sear.. Did you try using both lowers on your Grendel upper?

Did not try other lower. Upper returned to vendor and saw video of smith shooting it on their lower. Problem seems to be the difference in the BC weight. I will have to try it with the Colt supplied BC that weighs less. Then will try supplied auto BC with 556 upper to see it it functions there.

Bolt just not coming back far enough to catch bolt stop or to go back far enough to get behind rounds loaded in mag.


Need more info..

Answers to questions inserted in your post.

Smith believes Colt lower is the issue. If needed, I guess I'll have to build a lower comparable to the one the smith used.

[montana]

If the bolt carrier can be cycled by hand with an empty mag and is caught by the bolt catch without a problem, then you need to look at other areas.. But first separate the upper from the lower and then insert the bolt carrier into the buffer tube until it stops in the buffer tube and measure the distance from the back of the carrier key to the receiver..There should be at least 2 quarters width clearance. Check video below.. The bolt carrier should be 6.667 to 6.677 in length minus the bolt..I'm assuming you already tested the gas rings..
I have seen only one buffer tube that I needed to replace that was out of spec. It wouldn't allow the bolt carrier to cycle far enough back and it would barely pass the bolt carrier catch at full cycle..If your bolt carrier is at proper length, I would try another bolt carrier to see if it behaves any different. Every lower will have different friction values such as the type of trigger, buffer tube quality, mag placement in the lower, etc. Did you do the recoil spring-buffer inspection? Here is a good video of these test..https://www.youtube.com/watch?v=kvfr4Xb3wLY&t=29s
Just do the process of elimination with your lower and you should be able to find the problem if your upper functioned fine with the gunsmiths lower..
If all things being equal and the lighter bolt carrier seems to fix the problem, then your rifle is already running on the edge of short stroking..Your rifle should run equally reliably with a standard weight bolt carrier..

[Klem]

Vag,

Sounds like you are on the edge of being undergassed.

If everything works fine, except when using the heavy A2 rifle buffer, then maybe the available gas does not have enough pressure to cycle the full distance required for a mag catch.

A 5oz A2 rifle buffer is equivalent to an H3 carbine buffer - the heaviest you can get. There's not the carbine range of rifle buffers but JP does sell a lower weight A2 buffer which is 3oz.
https://www.brownells.com/rifle-parts/re...27116.aspx
Obviously the tension in the spring would also be part of the equation but a 3oz rifle buffer is about the weight of a MILSPEC carbine buffer.

FYI, on another forum people have weighed difference between SA15 and full mass FA16 carriers. It's anything between 0.2-0.7oz depending on manufacturer.

You can also look at bleeding more gas or changing out your powder if you hand-load.

That's if it is gas and not a faulty lower.

[vaguru]

Thanks guys for the information. Soon as I get time I will test with BC that came with original rifle from Colt. Doubt lower is faulty as this rifle configured for High Power service rifle has shot thousands of rounds of 223 in the past.

My gut feeling is that it is on the verge of under gassed. Don't want to enlarge gas port, so may have to build a lower that duplicates what the smith used to test.

Didn't really give powder a thought, but that makes sense as well. Unfortunately, factory ammo functions same way. I will have to measure hammer resistance of the 2 stage trigger installed vs a mil spec fire group.

One way or another, new lower or not, I will get this to function as it's an accurate barrel.

[LRRPF52]

Biggest piece of information missing is the ammunition.

What ammo are you shooting?

I have built and test-fired quite a few 20” 6.5 Grendels, including ones with AA, Satern, Lilja, Bartlein, and some other barrels I’m not thinking of right now.

All have had .094” ports, which is no different than .096” in practice. (The 3rd decimal is in the weeds.)

I have tested with 5.2oz AR-15 rifle/Rifle RET/Rifle spring, A5/A5 RET/longer spring, H2 buffer/Carbine RET/Carbine Spring, H Carbine/Carbine, and 2.9oz carbine buffers.

All of them fed, chambered, fired, extracted, ejected, and cycled through the entire cycle of operations repeatedly without issue.

The 2.9 oz carbine buffer set-ups seemed to have a little more 3 o’clock ejection and felt faster with cyclic rate, but not too much difference.

H2, A5, and Rifle set-ups seemed to work more ideally.

If your ammo is a light load, that will do it with the RLGS and .09x” port. 90gr TNT factory ammo even is hard to get to run in RLGS 20”.

107gr SMK not pushed hard enough could also cause short-stroking/failure to lock on LRHO.

Another possibility is an inefficient bolt carrier with loose fit to the bolt’s gas rings.

Do the bolt gas ring tests, both of them, and see if it collapses.

1st test is fully assembled bolt, bolt extended, set face down on flat surface. If it collapses, you have a loose carrier/bolt flange and gas ring set-up.

Next test is just the bolt in the carrier without the cam pin, firing pin, and FPRP.

Not all carriers are created equally, and the 3 bore inner diameters are critical for optimum gas pressure containment for the operation of the system:

[vaguru]

LRRPF52,

Thanks for posting in this thread.

Ammo used was PPU 110 gr factory and some handloads using 120 gr Hornady ELD Match bullets. Loads were H335 26.0 gr, 26.5 gr and 27.0 gr. The27.0 gr load chronographed much higher (major velocity spike and high ES) than the lighter loads and appeared too hot. Also tried W748 with 28.5 gr, 29.0 gr and 29.5 gr. These loads showed normal velocity gains (+20.5 fps per 1/2 gr increase) with 29.5 gr load showing 2576.2 FPS AV and 23 ES. All these loads in new Hornady brass and WSR primers as listed in both Hornady and Sierra manuals.

Loaded one box PPU brass once fired, H335 load of 26.0 gr same 120 gr ELD Match, 2529.2 FPS AV and ES 5. The factory PPU 110 grain was 2804 AV with 23 ES. Only 2 rounds locked back on empty mag as I single loaded each round while shooting to keep barrel temp down and to record velocity. One was the PPU 110 gr, and the other was a 29.5 gr W748 load.

The gunsmith tested with Factory Hornady 123 gr SST. I haven't found any of that to test, yet.

I did the gas ring test early on as I thought about that first. Assembled BCG stands on bolt face without issue, and just bolt as well. Bolt with rings has what I feel is good tension throughout travel.

One thing I did notice however is that the BC chamber is not chromed, rather Nitride. I questioned gunsmith about that and was given his explanation. I will have to live with that.

I will do some further testing when time and weather permits.

[vaguru]

Upper received back from gunsmith today. I noticed that the BC was changed as per my request. I then weighed the BCG and the BCG from the semi auto version Colt lower. There was a 1 oz. difference, the Colt was the lighter of the two, as expected. I added a little more lube to the BCG ( the heavier one) that came with the Grendel upper and proceeded to fire and function test. I did take both BC's with me, just in case.

Loaded one round in the mag and fired. Proper ejection and bolt locked back correctly. Tested 3 more shots that way, all locking back correctly on the bolt. Loaded 3 rounds for semi auto fire, knowing that it would function correctly since it locked back on an empty mag as it should. It functioned as expected feeding reliably and locking back on the bolt when empty.

Nothing wrong with the Colt Sporter lower, or the 5 oz rifle buffer in the A2 stock assembly. Apparently the issue was caused by a faulty BC. The gas chamber didn't look correct to me upon inspection, having an area about mid length of the chamber where material appeared torn out ( galled depression) rather than machined.

Called the smith and relayed my results and satisfaction with returned upper. The BC that was sent back with the upper was the one he tested with as well.

In conclusion, I now have what I expected to get initially, a fully functioning Grendel upper that functions with the lowers I have. Now to continue with load development and get some shooting in, on paper and maybe some coyotes too!

Thanks to all that responded to my original post, I appreciated the help.

[LRRPF52]

Great to hear you got that figured out.

Bad Bolt Carriers that don’t meet the TDP specs for the 3-bore inner diameters are often the culprit that people have a hard time identifying without armorer inspection gauges.

A lot of nitrided carriers have been machined “to-spec” but that spec is set based on the assumption that they will be finished with hard chroming inside the bolt bore, so with nitriding, they end up looser and don’t seal the gas system as tightly.

Colt, LMT, BCM, KAC, and only a few others follow that chrome-lined TDP closely.