Adams Arms piston overgassed or extractor/ejector issue?

[Arizona_Mike]

My piston upper is throwing LC 193 and LC 855 off the leading corner of the deflector to 1:30 (forward). Brass has a line or light dent between 1/4 and 1/3 up from the base (body section only).

I am going with their low mass carrier and for that reason do not want to add buffer weight (defeats the purpose). I'm running a 1 oz buffer (empty C).

Barrel is a 16" Rainier Arms select MEDCON, mid length.

I am shooting with the 5 position (Off, 1/4, 1/2, 3/4, Full) gas valve at 1/2. At 1/4 it does not reliably function and never locks back on the LRHO. This and the fact that is is a middy makes me assume it is probably not overgassed in the classic sense.

Mike

 

[MagnumWill]

According to this, you would want a HEAVIER bolt carrier. If you make it lighter, all the gas system is going to do is throw it back even harder if you lighten the reciprocating assembly. I vote an M16 bolt carrier (if it works with the piston conversion, that is), and if that doesn't solve it an H buffer. I don't think you're overgassed, i think your reciprocating assembly is underweight.

 

[wally]

If you don't want to add mass, try an extra power buffer spring, but adding mass will be more effective. What's the point of the "low mass" carrier?

I always though ~2 o'clock was perfect ejection for an AR so lefties don't take some in the face.

I assume that its the dents in the empties from the brass deflector that is your real complaint.

 

[briansmithwins]

Is there a problem with functionality? If the rifle is running 100% I wouldn't mess with it.

The chart can be useful but it's also led many people to mess with a perfectly running rifle.

BSW

 

[Arizona_Mike]

First of all I need either the standard Adams Arms or low mass Adams Arms BCG designed for short stroke piston. They do not offer a gas key replacement.
Their low mass carrier is only about 1.8 oz lighter than the an M16 carrier, heavier than many "civilian" carriers out there.

It is their system sold with their BCG designed to work together. I've only reduced the reciprocating weight by 1.9oz further.

Given the design, the fact that it is a middy and not carbine gas ,the fact that Rainier is not known for large gas ports, and it is clearly undergassed one click down, Adams Arms thinks is might be weak outward ejection from still being under-gassed and has asked me to try opening the gas valve on the piston further.

This might be the case as the dents are much smaller than I see in pictures form overgassed guns and there are no signs of extractor damage to the rims. The rims are smoother than my DI middy with a 12oz BCM M16 BCG with an adjustable gas block.

Yes, my complaint is the dented brass. The range I am a member of won't let you pick up brass forward of the concrete but I usually remember to bring my brass catcher.

Mike

PS: All my other ARs are DI and I tune most of my guns to throw to 4:00. With Syrac gas blocks this is usually about 1/4 to 1/2 turn further than what is needed to reliably operate the LRHO.

 

[wally]

PS: All my other ARs are DI and I tune most of my guns to throw to 4:00. With Syrac gas blocks this is usually about 1/4 to 1/2 turn further than what is needed to reliably operate the LRHO.

Then I'd suggest trying a heavier buffer if you can't tweak it with gas adjustments.

I keep spare H, H2, H3, and Spikes' 9mm buffers on hand for such testing/tweaking. Easy to do at the shooting location too.

I'm usually running H or H2 buffers for better operating when suppressed and don't find the 4-5 o'clock unsuppressed ejection to be an issue, but then I don't shoot 'em lefty

 

[Fishbed77]

is there a problem with functionality? If the rifle is running 100% i wouldn't mess with it.

The chart can be useful but it's also led many people to mess with a perfectly running rifle.

This. ^^^

 

[Arizona_Mike]

Agreed but I reload

 

[MikieG]

I am currently building two 7.62x39 guns using Adams pistons. Midlength barrels. The x39 is low pressure and low velocity. It needed a .96 gas port. Empty buffer piston weighing 2oz down from the 4.5oz it was. And lastly 6 coils cut off from standard carbine buffer spring.
I am awaiting reduced weight carriers from Adams right now. Hoping that going from 9.5oz standard carrier down to the 7.1oz light carrier will allow an uncut spring to work. We shall see.

 

[Arizona_Mike]

I still need to either get to the range or get to the desert. Still hoping I was undergassed and not throwing far enough to the right to clear the receiver.

Mike

 

[MikieG]

At first i put it together D.I.. But with the low pressures and lack of gas, i was having alot of problems. No matter what i tried it just couldnt Cycle.
I changed to the piston. Then,with less volume to fill, suddenly it was very close.
Never once considered how much gas volume you need to fill a D.I. system.

 

[Jeremy2171]

Agreed but I reload

Why is dented brass an issue?? That dent will come right out next time you shoot it...

 

[MikieG]

I am not too worried about that. Right now i am trying to bring the system into balance. My goals are a gun that will run the weak surplus ammo, and with the 60% gas block setting, be able to run hotter hand loads.

 

[carbine85]

A mid length with a standard buffer and proper gas should run almost any ammo and throw brass to the side back. I don't your problem is in the carrier. AR brass is always subject to dents.

 

[Arizona_Mike]

A mid length with a standard buffer and proper gas should run almost any ammo and throw brass to the side back. I don't your problem is in the carrier. AR brass is always subject to dents.

I really don't think standard AR behavior is at all applicable to a piston upper.

I have never had any trouble getting any of my DI ARs (5.56, 300 BLK, 7.62x51) to throw to 4:00.

Due to rain this weekend (an amazingly absurd amount of rain) I was not able to test other gas block postions. I'm going to keep my AR in my car for a few days in case I get a chance to swing by BLM land or the range after work this week.

Mike

 

[Bartholomew Roberts]

I really don't think standard AR behavior is at all applicable to a piston upper.

I'd agree. The chart above isn't all that relevant to this particular problem. It is also one reason I stick to DI ARs - if you have a problem with one of the pistons, the knowledge base you can tap into is much smaller and you'll get 20 replies that are relevant to DI ARs, but may or may not be relevant to your piston AR.

 

[briansmithwins]

I'd agree. The chart above isn't all that relevant to this particular problem

The problem is that this isn't a real problem.

The OP wants a sports car that hauls 3 cubic yards of gravel while getting 80MPG.

BSW

 

[Arizona_Mike]

That's a little dramatic. I want my brass a little less beat up and don't want to loose too much anodization and metal where the front end of the deflector meets the body of the upper, but I agree it is not a big issue, just a detail.

If fine tuning ejection is a subject that holds zero interest for you, no one is forcing you to follow this thread let alone comment.

Mike

 

[briansmithwins]

That's a little dramatic.

Sorry, I wasn't trying to turn the thread into a soap opera.

I do think you're trying to do something that's just not that realistic. Everything is a compromise and to get reliable function with a self loading rifle you need a certain minimum energy in the bolt/bolt carrier. Some of that energy is going to get transferred to the empty cartridge case, and it's going to bounce off the rifle on its way out, resulting in dents to the brass.

You could solve your brass problem by turning off the gas and converting the rifle into a manual straight pull. If you want to retain self loading I think you're going to have to accept some level of damage to the brass.

BSW

 

[Arizona_Mike]

Hauling 3 cubic yards of gravel in my sports car.

Managed to get to a nice spot on BLM land on my lunch break.
Dry dirty and 1/2 open: most cases dented.
3/4 open: 5/5 cases clean.
Full open: 5/5 cases clean.
Did not pack lube but I smeared some thick dino juice from the underside of my 710 cap on the outside of the BC.
1/2 open again: 1/4 cases dented.
3/4 open: 6/6 cases clean.

Conclusions:
1. I was undergassed, not overgassed. Cases were not being thrown far enough right to clear the receiver. I can now see smears where they are hitting the deflector flat rather than the leading sharp corner.
2. This short stroke piston upper throws forward no matter what.
3. Previous experience with 1/2 open was clean and lubed.

Mike