Piston BCG question

[Uffdaphil]

Based on my piston MRP’s BCG having no gas key or rings on the bolt am I to assume that friction fit is not a thing? I put in a POF roller cam pin and the bolt just flops back and forth. The original pin does the same. Just normal piston perversity?


P.S. I love the fixed firing pin retainer.

[UWone77]

My Adams Arms upper has a spring in the carrier that doesn't allow it to flop back and forth.

[Uffdaphil]

Further searching makes me think the floppy bolt is normal for the piston MRP.

[Stone]

What does MRP stand for? Also, what rifle is it, who makes it? I have the answer to your question but need the info before I answer it... ETA: The LMT Monolithic Rail Platform (MRP™) ??

[Uffdaphil]

Here is the 16” piston upper. Mine has a the Shovelnose rail

https://vtsupply-com.3dcartstores.com/LMT-16-MRP-556-Piston-Top-End-Lewis-Machine-Monolithic-Complete-M-LOK-Rail-Upper-Receiver-AR15-AR-15-MLOK_p_4571.html

[Stone]

Honestly I would ditch the POF cam pin, in theory it may sound good but the reality is that LMTs engineers chose the standard pin for a good reason. Cam slot wear is necessary for a good mating of your BCG to the upper receiver which in turn creates less BCG movement and keeps the tolerances tighter and makes cycling more proficient in a productive manner. This argument was debated on the LWRC forum ad nauseum with the engineers at LWRC chimming in and explaining that adding a variable (AKA the POF cam pin) was not helpful in any way. Keep the cam pin channel well lubed... I use SLIP EWL first then a light coat of SLIP EWG spread on top of it. Its like the cam pin is riding on ball bearings. The guy from POF is a salesman and he wants to sell his product... If you really want to reduce friction use the lube method I mentioned and Cryptic coatings mystic black cam pin...

[Uffdaphil]

Thanks, Stone. I’ll stick the roller cam in my cheapest M4gery and replace with FCs Sotar NP3 standard profile dimple pin.

[Stone]

Is there a coating on LMT's cam pin? Like NP3 or is it just a phosphate finish? Also I added a little more to my post above... I have alot more to add to your original question but am a little busy today and will chime back in ASAP...

[Uffdaphil]

The FC pin comes in NP3 or nitride. I want to use my NP3 for the dimple so I can always insert in the same direction. The LMT carrier appears to be NP3. The black cam pin looks too smooth to be phosphate.

I also dual lube the cam pin - with Slip or Weaponshield and EWG or ALG very thin grease.

[Joelski]

I would add to Stone's advice that the very tippy-tippy-top tier manufacturers like LMT (and obviously KAC, LWRC, and H&K) have probably sunk some R&D bucks into every nook and cranny of that fancy rifle to the extent that "Mil-Spec" and "TDP" are akin to amature hour gunsmithing. I'd be hard pressed to replace or otherwise "upgrade" anything on such a rifle, not that FCD wouldn't be a great addition, just less likely to be better than the stock part.

You could always have a mark engraved on the LMT cam pin. Prestige man, PRESTIGE!!! [:D]

[Stone]

To gauge an accurate starting point in this discussion I would email LMT and ask them why they chose not to use gas rings on that bolt that comes on that specific rifle. Once you get that intell we can take a more educated dive into the dynamics... Joelski is right, those companys usually run their rifles systems to the point of failure and make necessary changes as they go along...

[Uffdaphil]

I could engrave the WEVO symbol, but then the pin balance would be off center and would shake like a 3 legged Maytag! Probably blow through the upper like a funny car piston! The similes are endless.

I may have used my last FC cam pin so put back the original and calling it good. Not gonna email LMT unless a problem pops up.

Thanks for all the info.