I have a 1942 Mosin Nagant 91/30 rifle. I was shopping Ebay and found a muzzle brake that fit on about the same way the bayonet mounts. Slip on barrel, twist around front sight base. This brake doesn't have the spring plunger to lock it on but uses three brass set screws.
Howling Raven was the brand and was about $35 shipped.
The quality of this item was surprising. The flat areas were flat. The vents crisp. Every thing had the sharp edges rounded slightly. Very well done for a $30 item.
The rifle when shot off hand the muzzle rose up about 6 inches. Had a gentle nudge on the shoulder. With this brake the muzzle dips about 2 inches and the nudge isn't as noticeable as without. I was impressed the this item did this well.
I didn't shot it enough to know how much effect it may have on accuracy.
These rifle were sighted with the bayonet attached. I shoot without so it tends to shoot high. This item weighs 10 ounces, the bayonet weighs 11 ounces. Just the weight may pull the POI closer to the POA.
I may not get to shoot it again for awhile but will report my findings as I get them.
Leo

Thanks for the review!

you may already know, some nagant sights are calibrated in arshin, not yards or meters -- and, iirc, are setup to generally shoot zero well past 100 yards/m ... i think it's about 230, but that's likely the wrong number

i've used a like brake on SKS -- it was fun, really loud, and somewhat effective