Suppressed build - No last round bolt hold open

Supermatch

Supermatch

That's my purse!
Life Member
Multi-Factor Enabled
Joined
Jun 29, 2022
Messages
3,723
Location
NC
Rating - 100%
9   0   0
Took the new build out today - DD RISIII 11.5" upper, LMT MARS lower, Velos low backpressure can. Buffer is an Aero carbine buffer tube, carbine spring, and H3 buffer. Three different ammo types with ejection patterns from 2:00 to 3:30ish. Every round fed perfectly, but there was no last round bolt hold open, suppressed or unsuppressed. I can manually lock it back without issue, and the bolt will travel about 3/16" beyond the catch with a mag inserted, so its not a matter of bottoming out the spring. I don't see any unusual indicators of wear, etc. on the bolt catch.

Thoughts? Maybe I need to go down to an H2 or H1 buffer, but I would think that I'd be seeing under-gassed ejection patterns if that were the case? Not wanting to go to adjustable gas blocks or oddball buffer setups (e.g. JP). Scouring the internet I've seen some mentions of issues with LRBHO on ambi lowers due to the added weight of the ambi bolt releases, but not sure there's really a fix for that (though JP says to go lighter on the spring in that instance).

LMT tech support suggests their spring with an H2 buffer, so I'll likely give that a try first.
 
barf said:
Hate to ask the obvious, but I’m assuming you tried multiple mags with the same results?
Click to expand...
Yes, 4 different mags.

DS1911 said:
Why are you wanting to stay away from adjustable gas blocks? It’s pretty much the easy button for solving gas gun issues with a can.
Click to expand...
Reliability and ability to replace everything with OE spec parts if need be. Just don’t want to deal with position screws locking up from carbon, etc. I’ll take a little extra recoil for the sake of reliability with this one.
 
Giving the H2 and LMT spring a shot. Hopefully it’s just short stroking a touch and the ejection pattern is just deceiving me.
 
Yeah I'd go for the H2 first to try that out.

I had an issue recently with an 11.5 build that was gassed great for unsupressed.... but the bolt velocity was actually too high for lock back when supressed (gra ted this is with a Sandman S), ended up dropping in a bootleg adjustable bcg which turned that build into one of the softest shooters I've ever owned.
 
Cole_McD said:
Yeah I'd go for the H2 first to try that out.

I had an issue recently with an 11.5 build that was gassed great for unsupressed.... but the bolt velocity was actually too high for lock back when supressed (gra ted this is with a Sandman S), ended up dropping in a bootleg adjustable bcg which turned that build into one of the softest shooters I've ever owned.
Click to expand...
This was definitely a soft shooter both with and without the can and just the H3. I plan on running it suppressed only, but want it to function with both the low back pressure can and a Lahar 30 once it’s out of ATf jail.
 
Supermatch said:
This was definitely a soft shooter both with and without the can and just the H3. I plan on running it suppressed only, but want it to function with both the low back pressure can and a Lahar 30 once it’s out of ATf jail.
Click to expand...
Dude I would love a write up on the Lahar once you get it! I've been seeing them go for crazy cheap and they seem like a great option.
 
Cole_McD said:
Dude I would love a write up on the Lahar once you get it! I've been seeing them go for crazy cheap and they seem like a great option.
Click to expand...
I’ve got two of the regular lengths sitting at THSF. Jumped on them during the primary arms sale when they were something like $500. The other one will live on a .300 blk pistol.
 
Adjustable gas blocks only work with issues like these if you open up the gas port to allow more gas in the first place. It should be in the .070" range . I just opened up a 10.3" FN barrel from a .063" port to. .071" port and it turned from a miserable running gun into a sewing machine,
 
wired said:
Adjustable gas blocks only work with issues like these if you open up the gas port to allow more gas in the first place. It should be in the .070" range . I just opened up a 10.3" FN barrel from a .063" port to. .071" port and it turned from a miserable running gun into a sewing machine,
Click to expand...
I believe this barrel is in the .073 to .075 range.
 
Supermatch said:
I believe this barrel is in the .073 to .075 range.
Click to expand...
If you are short stroking with a port that big and an 11.5" barrel start working on lighter buffers. Light spring guns are their own circle of hell though although sometimes guns start off with the wrong spring to begin with . Woork with the buffers first but make sure you have a standard spring rate buffer spring in there first.
 
Supermatch said:
Maybe I need to go down to an H2 or H1 buffer, but I would think that I'd be seeing under-gassed ejection patterns if that were the case?
Click to expand...

IMO, yes. Bolt can be going too fast, or the bolt catch can be worn/screwed up.
It is also possible the cases are bouncing forward off the shell deflector, falsely indicating an overgassed condition.

I've found that getting a buddy to do some slo-mo video of port area can often help a lot.
 
Supermatch said:
I hadn't considered that, but its a good point. Thanks for pointing that out.
Click to expand...
Since you generally always use a shell deflector unless you're doing an A1 build that doesnt really mean much. The ejection pattern guide takes that into account
 
wired said:
Since you generally always use a shell deflector unless you're doing an A1 build that doesnt really mean much. The ejection pattern guide takes that into account
Click to expand...

Not always, IME.
 
I don't have as much experience as many of you, but I've never put much stock into the various ejection direction infographics you find around on the internet. It seems to attempt to over simplify something without accounting for all the variables.

Sort of like those clock targets that tell you you're pushing or pulling or anticipating.... I don't like those either.
 
OK, back to the range today.

LMT lower, now with their recommended/standard issue H2 buffer and carbine spring. No LRBHO. Swapped in H1 buffer, no dice. Swapped in spare carbine spring and standard carbine buffer, no dice. Only change is ejection patterns getting further forward with lighter buffer weights.

Swap spare Sig 16" upper onto LMT lower, no dice.

Swap DD upper (from this gun) onto a Sig lower with an H2 buffer, locks back no issue.

So the upper from this gun was confirmed to function without issue on another lower with zero problems. Yet this lower refuses to lock back with two different length suppressed uppers, regardless of buffer weight. It will lock back with an empty mag no problem.

Is there anything left to try before I send this thing back to LMT to see if something is out of spec? I'm hesitant to try something like a lightweight spring, because the upper is already patterning on the overgassed side and had no troubles cycling an H2 buffer on a different lower.
 
Spoke with LMT again this morning. They told me to try a Gen 3 Pmag specifically. The u-shaped notch on the Gen 2's as opposed to the square notch on the back of the Gen 3's appears to cause enough additional drag with the added mass of the ambi bolt release to not overcome the spring force quickly enough to catch the bolt. This is also part of the reason why the original MARS design used steel, and they later switched to aluminum on the MARS-L just to get the mass of the ambi controls down. So I will try a Gen 3, and if it works, will take a file to all of my Gen 2's to open the notch up to match the Gen 3's, and will only buy Gen 3's moving forward.
 
Supermatch said:
Spoke with LMT again this morning. They told me to try a Gen 3 Pmag specifically. The u-shaped notch on the Gen 2's as opposed to the square notch on the back of the Gen 3's appears to cause enough additional drag with the added mass of the ambi bolt release to not overcome the spring force quickly enough to catch the bolt. This is also part of the reason why the original MARS design used steel, and they later switched to aluminum on the MARS-L just to get the mass of the ambi controls down. So I will try a Gen 3, and if it works, will take a file to all of my Gen 2's to open the notch up to match the Gen 3's, and will only buy Gen 3's moving forward.
Click to expand...
Weird. And kinda lame that they don’t work with any/all mags. Oh well. Easy enough fix if it works 🤷🏻‍♂️
 
Last edited:
Supermatch said:
Yes, 4 different mags.


Reliability and ability to replace everything with OE spec parts if need be. Just don’t want to deal with position screws locking up from carbon, etc. I’ll take a little extra recoil for the sake of reliability with this one.
Click to expand...
I not tracking with your thought process on the adjustable gas block. You are running a lower with proprietary parts that cannot be replaced with mil-spec parts. So why not add the adjustable block since mil spec parts are already a problem.

I know some people argue the carbon build up locking the screw in place etc. But if you buy a quality gas block, put anti-seize on the screw before it’s installed on the gun you will be ok. Plus if this gun is going to dedicated to that can, once you get it tuned you won’t be turning the screw anyways. Just my thought process.

LMT makes a solid rifle mil spec rifle. But if the MARS lower has to use specific mags, to me that’s a whole other issue. A lower that cost that much should run with every magazine made to mil spec, again just my $.02. I’m lot bashing LMT, like I said they make a great mil spec rifle.
 
Average Joe said:
I not tracking with your thought process on the adjustable gas block. You are running a lower with proprietary parts that cannot be replaced with mil-spec parts. So why not add the adjustable block since mil spec parts are already a problem.

I know some people argue the carbon build up locking the screw in place etc. But if you buy a quality gas block, put anti-seize on the screw before it’s installed on the gun you will be ok. Plus if this gun is going to dedicated to that can, once you get it tuned you won’t be turning the screw anyways. Just my thought process.

LMT makes a solid rifle mil spec rifle. But if the MARS lower has to use specific mags, to me that’s a whole other issue. A lower that cost that much should run with every magazine made to mil spec, again just my $.02. I’m lot bashing LMT, like I said they make a great mil spec rifle.
Click to expand...
You can swap a mil-spec bolt release into the lower if needed, you just lose the ambi capability. If I had known it wouldn't lock the bolt back with a Gen 2 Pmag, I'd have gone a different route. I'm very frustrated with it in that regard, especially given the price. If it works with a Gen 3 or a steel mag with an anti tilt follower, then that's a tradeoff I'll have to live with for now. Maybe I'll get lucky as the parts wear in.

In either case, the issue has nothing to due with gassing, so replacing the gas block gets me nothing. It's already a very soft shooter, and I've noticed no gas to the face that would want me to change blocks. Ejection is close to perfect unsuppressed, and is only slightly overgassed when suppressed, which is how I'd like it to be from a reliability perspective.
 
If you need to borrow a Lancer or metal magazine for troubleshooting I've got some.
 
Ok, back to the range today. Three test runs - Gen 3 Pmag, Gen 2 Pmag with the rear notch filed square, and a Lancer mag borrowed from @Timfoilhat. Note that Lancer mags are square cut in the back much like the Gen 3 Pmags are, just with a slightly narrower cut.

All three locked the bolt back with both the much gassier K can, as well as the low back pressure can.

So from now on I’ll just buy the Gen 3 Pmags, but will spend a rainy day with a square file to update the bulk of my Gen 2s.

But LMT does seem to be correct in their assessment - the MARS-L lower just can’t handle the added drag from the plastic on the back of the Gen 2s.
 
After filing down the gen 2 pmag are you having any issues with the feed lips spreading out? @Supermatch

I've had lancers puke rounds when dropped wasn't sure if the cUs+0m pmag still retained rounds well.
 
Squanchy said:
After filing down the gen 2 pmag are you having any issues with the feed lips spreading out? @Supermatch

I've had lancers puke rounds when dropped wasn't sure if the cUs+0m pmag still retained rounds well.
Click to expand...
I can't say from a long-term storage perspective, but it seems no more or less flexible than before trimming. Compared to another intact Gen 2, manufacturing differences between production runs seems to have a bigger impact than the cut does, as the slightly shinier, uncut Gen 2 seems a bit more flexible than the one I had trimmed.

I just did a "shake the crap out of it" test on my lunch break, as well as a handful of drop tests. It spit one round out one time when it dropped right on the bottom corner of the mag. I couldn't get that to repeat. Dropped about 10 more times from varying angles, and no rounds came loose.
 
That's good 1 round is expected it's those 5 or 10 round pukes that are an issue
 
You must log in or register to reply here.
Back
Top Bottom