[SiliconeSword]

That's about where we've settled I think.

I also recommend checking the O-ring groove for any weird spots, and smooth it down with sandpaper up to about 400 grit.
Also, what grease are you using?
At lower power its sometimes hard to overcome the friction of the O-ring, I've seen a power gain when using Tamiya CeraGrease.
You can use others like TechT, light weight silicone, light weight motor oil, and whatever everybody else says, but when rubbing Tamiya between your thumb and finger while pressing hard, they will fly apart more violently than any other lubricant I've tried.

 

[SiliconeSword]

If you send me your barrel I can lap and R-hop it quite cheaply, I'm sick of seeing you struggle lol

 

[SiliconeSword]

The issue with the M40A3 is that the receiver is PLASTIC, hence why is has 4 trigger screws, 2 side by side on each end, so I don't think a VSR trigger would work out too well.

I'd maybe look at an ESC 90° conversion, but I believe you mentioned that the big sear and possibly trigger aren't compatible.

Maybe you could get a Mancraft SDiK for the APS2 platform lol, that would probably be best if you want hardcore performance out of a lame base, but that is likely not within a reasonable budget for you.

 

[SiliconeSword]

Yeah, that was an extreme downer, same with the barrel being a slip fit with no threads at all.
Tbh the Double Bell VSR-10 I had for a week had the same slip fit outer barrel, but at least that had a metal piston, full piston seal, 500 FPS, and a nice bolt pull with a GOD DAMN METAL RECEIVER.
For the $130 or whatever it was, it's definitely worth paying the extra $30 for it due to the stainless cylinder and all the other things that made it better.

 

[SiliconeSword]

I bought mine in 2017 or 2018 I think, so they made have gotten better, but I doubt it.

 

[SiliconeSword]

My two issues with the rifle were the nozzle and the hop setup, as the nozzle was too short and placed the BB in a bad place leading to sub optimal accuracy and horrible air seal, and the bucking and chamber were their own type of semi proprietary design and dimensions, along with the barrel.

If you are able to install the barrel and possibly lap it, do that, and if you can somehow either push the chamber back or slide the cylinder/nozzle forward that would be quite good too.
Idk if you've seen the thread I did on the M40A3, but it's in my signature and has a number of other things that I suggest you do.