Piston Conversions and Upper/Lower Fit

[krinko]

I did a CMMG piston conversion on an M&P in 5.45. The upper/lower fit allows for noticable play between the units, which I was assured-at purchase-was all "milspec" and proper. (It was the only 5.45 in town.)
During routine maintenance, I found aluminum shavings in the lower and discovered the bolt carrier was abrading the lower right quadrant of the buffer tube.
Installation of this rubber bumper under the takedown pin secured the relationship of upper to lower and a test session Thursday showed no further damage to the buffer tube.

So, how are piston conversion kits working on other loose rifles? Any shredded aluminum?



-----krinko

 

[HorseSoldier]

Just demonstrates the intrinsic flaw of piston conversion ARs -- the piston bolt travels under different force than the proper direct gas bolt, with a downward component that leads to wear and tear on the weapon exactly where you've observed it. Tightening up the fit fit might reduce the amount of wear and tear but the only thing that will eliminate it is going back to the direct gas operating system the rifle was designed for.

 

[MrCleanOK]

The infamous carrier tilt. One of many reasons piston ARs are inferior to DI guns.

 

[MCMXI]

So, how are piston conversion kits working on other loose rifles? Any shredded aluminum?

krinko, I don't know about the piston conversion kits, but the buffer tubes on both of my POFs are looking good at the moment. The P415 has about 1,500 rounds through it and the P308 about 500 rounds. The fit between the upper and lower of the P415 is very good, the fit on the P308 isn't as good but I'm working on improving it.

A couple of thoughts on this. First, why not put a blob of grease (Mobil 1 perhaps) at the location where you first observed the wear. Another thing to consider is filing a radius or a chamfer on the rear (bottom) of the bolt carrier where it first makes contact with the buffer tube. Anyway, you make an excellent point about the fit between the upper and lower and good for you for finding a solution that might improve accuracy as well as reducing or eliminating wear. This isn't just a gas piston issue either and the worst case scenario is that you have to buy a new buffer tube after a few thousand rounds. You'd better start saving now since those buffer tubes are awfully expensive!!

As for the posts by HorseSoldier and MrCleanOK .... yawn!!!

 

[krinko]

1858,
There is a dedicated bolt carrier supplied with the CMMG kit. It has the built-up pads, beveled edges and such to prevent damage to the buffer tube.
Now that the tube and bolt carrier stay lined up, it can perform as advertised.

Greasing the area had occured to me---and I might do it yet.
Didn't want to for the trial run as it might obscure the machinist's blue.
-----krinko