Project Überlight: sub 4lb SBR; sub 5lb suppressed SBR.

[Jerry R]

Very nicely done Sir - and a great write-up !!

[adh]

This is what the custom build forum is supposed to be.......thanks for the effort!

[henschman]

Great build and parts review. I have bought or am planning on buying a few of those parts myself, so it was good to have your take on them. I just paid for and sent off paperwork on a SiCo Omega this week. They are definitely amazing cans. It is almost unbelievable how a can that small and light will handle up to .300 Win Mag. I am also planning on getting the Spikes rear sight. I need a very low profile option to clear the ocular housing on my scopes (I mount mine a little lower than most so I can have a solid cheek weld). Also, call me old-fashioned, but I like having an elevation-adjustable rear sight with consistent click values. I have two of the KAC 2-600s, and while I love them, they are too high profile to work on my scoped ARs. They will stay on my red dot equipped rifles. Also I am planning on an RM06 for a Glock.

[NongShim]

I generally don't get down on stuff like this for myself but it looks an awesome setup. I really think those receivers are something! Beautiful machine work; very FAL-esque lightening cuts.

Even though it isn't exactly my cup of tea, I can seriously appreciate all the research and hard work that went in to this.

I am glad I found out about those receivers and the various low pro/light sights. Thanks for the effort.

[DekNgo]

Wow, outstanding job on the write-up. I thought I knew which direction I wanted to go with my ultra lightweight 5.56 build, but now I'm considering .300 Blackout...

Thanks for the great info!

[Colt guy]

Great Great Job.... Makes me want to put one together.

.

[Colt guy]

How did I miss this in June? must be working too much.

[DekNgo]

Any updates?

[jerrysimons]

Any updates?

Yes, thanks for asking. I meant to post earlier but haven't had time to shoot it in its most current configuration, since you asked I'll post the changes here before the range report.




I swapped some parts out. Some add weight but serve an improvement in function. Some save weight, and I kidd you not, the thing still weighs right at my target weight of 4.0lbs unsuppressed and is <5lbs suppressed. Overall I had a .37oz net weight gain doing the math on the parts weighed individually. With the .2oz NSR panel handstop not pictured in the original weight pic showing 63.5oz the gun weighed in at 63.7oz, with the new parts swapped in the scale shows right at 64.0oz, no trickery. I think I might still swap the V7WS titanium firing pin in place of the melonited steel pin to shave hopefully another tenth ounce and get down 63.9oz.

Weight 64.0oz/4.0lb:


I swapped the steel KMR clamp blocks (.35oz) for the V7WS titanium KMR clamp blocks (.2oz).

I swapped the original Magpul MOE-K grip (2.0oz) for an UCWRG G23 grip (2.35oz) because, as you can see in the photos below, the original moe-k grip and the 1st run 2A Armament Balios-lite lower didn't interface well. The overhang of the trigger guard tore up my middle finger when shooting. The G23 is still one of the lightest grips, interfaces flush with the integral trigger guard and just feels better all around. I made this same decision on Noveske Ultralight yet thought I would get away with prioritizing the weight over comfort on this build but the flesh missing from my middle finger protested.

Original 2.0oz MOE-K grip:


2.35oz UCWRG G23:


I swapped the V7 Weapon Systems .750 Titanium gas-block (.8oz) for a SLR Rifeworks titanium Sentry 7 adjustable gas-block (.97oz) mentioned earlier in the thread. I had it cerakoted black. With the insanely lightweight reciprocation assembly on this experimental gun gas-adjustability is crucial. I have shot the rifle since my last post but prior to the adj gas-block. I successfully tuned the adjustable carrier back a little more to where subsonic rounds struggled to cycle the action without a suppressor but supersonic rounds still functioned unsuppressed. I am not sure how much margine is left to adjust and there are other variables involved, mainly ammo type and temperature, but that tune helped slightly (it is still a lot) with the intense blowback I was getting while shooting supersonic rounds suppressed on the previous tune (cycle but no lock back with subs unsuppressed; full function with supers unsuppressed). Since this gun is a part experimental I thought adding the ti Sentry 7 was a perfect match for the build because I now have regulation at both the more conventional location of the gas-block and at the carrier with the adjustable carrier key while still fitting with compounding weight savings by being made of Ti (even though it is slightly heavier than the non-adjustable V7 Ti block). Things should be real interesting now! I can mess with the settings of each and really shake out the concept of carrier regulation vs gas-block regulation and probably end up with a really nice tune at the end! Also, if I were to swap out the superlight weight reciprocating assembly for a more standard set-up I will still have reliable gas regulation (the SLR Rifleworks Sentrys are excellent!).

.97oz Titanium SLR Rifleworks Sentry 7 gas-block cerakoted black:


Did I mention this build was experimental? Some bad bolt wear is an issue that came up and I really don't know what to make of it. The bolt looks like it is hitting the lugs of the barrel extentsion while slightly rotated counter-clockwise/ in the direction of locking. The deformation on the left side (facing muzzle-ward) of the bolt lugs indicates this. The only theory I have been able to work out is that the BCG being so light while traveling faster than designed with less gas venting from the expansion chamber (carrier key regulation), the BCG may be bottoming on the reciever extentsion hard enough to jolt the bolt and cam pin backward thus rotating the lugs a little toward counter-clockwise/locked. Then when the BCG accelerates fastly forward due to being extreamly light weight, it slams on the barrel extentsion lugs hard enough and at enough of an angle to deform the top left corner of the bolt lugs. This may be a big issue since I only have about 180rds down the pipe mixed supers/sub/suppressed. Against this theory is that there should not be much room inside the upper for the cam pin to rotate toward locked when out of battery and I don't see gouging on the left side of the upper reciever interior from the cam pin. I do however remember that the boomfab bolt carrier had a slight amount more play than the JP Lowmass it replaced. Perhapse that is where the give is in the rotation of the bolt interfacing with the barrel extentsion lugs. Though I suppose it could also be an undersized cam pin or oversized cam pin track allowing for the bolt to rotate but that, I think, would have enough play in it to not wreck the top left corners of the bolt lugs like is happening.

Any opinions? This is new territory for me, I have never seen this kind of wear on a bolt before.

Unusual bolt wear:

[Red falcon]

Awesome thread, thank you sir. I appreciate all the info on lightweight parts!