First of all, you probably do not have a real H&K MSG90. They were not produced until after May of 1986 and a real MSG90 from H&K has a swing down lower which makes the receiver qualify as a Machine Gun by the BATFE.
Secondly, if you have an H&K 911, SR9 or SR9T that you think is an MSG90, it does not have a factory threaded barrel. And you will ruin it's value if you do thread it.
Thirdly, if you have a clone rifle and you put a can on it, one of the most likely outcomes is that it will shoot like ass. As many of the clone rifles have shit barrels that have a bore that is not concentric with the exterior of the barrel. Meaning you would have to have the barrel threaded to match the suppressor or have a new end cap made.
I would find a different platform to suppress.
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<span style="color: #FF6600">hey hk professional befor you start sliceing and diceing why dont you get a few facts first.
my barrel is a douglas barrel 26" long it is threaded 15x1m by phil flack, iam not going into the details of my build already did a thread on that.
the gun is a half moa gun easy, so you can keep your negitive b/s to your self.</span>
Not being negative, but realistic. If your gun is 1/2 MOA dependably, I wouldn't touch it, because any of the experienced hands on this sight will tell you that a repeatable 1/2 MOA out of a gas gun is almost unbelievable...M1A/MSG90/KAC MK11 Mod O/etc.
Roller locking bolt design is good for about 1 MOA due to the way the barrel is pressed into a trunion which is pinned into a sheet metal receiver and has to have reinforcements welded into the sides of the receiver to stiffen it to improve accuracy.
Here is a pic of my PSG1 it is not threaded, the factory guns were not threaded. That is why the MSG90 was introduced to have a flash hider, and a swing down lower, so that lowers could interchange with the other .308 weapons and to correct some other deficiencies in the PSG1 platform.