I installed the same primary weapons brake on my 10 to 12 year old vector 51that has always run flawless.
But I have to say a little tough on recoil until I installed the brake and a bill Springfield enhanced buffer in the A2 stock.
When installing the brake I found I had to remove the internal crush sleave for the break to install all the way up on barrel
Properly.I was able to install completely by hand and spring clip keeps break in proper clocked position.
How old is that FSC91 of yours? PWS told me the older version had the crush washer, while the never version that I bought does not. I didn't see anything on the inside of my FSC91 that was removable, other than the retaining spring/clip.
I'll check out that Springfield buffer, too. He's done some FCG work on a CETME and a couple of FALs, always did great work. I'll have both the A2 stock and an A3 stock. Might be nice to have one for both.