So I recently purchased a bayonet for my PTR and was having some issues installing it and the lug mount part. I watched a video on YouTube on how to install the lug and on the video the guys dust cover thing that comes with the PTRs pops off seemingly easy. Mine won't move when I apply pressure to the set screw with an Allen wrench. Anyhow after I couldn't seem to make it work I decided to see how well the bayonet fit the flash hider and the ring seems just a tad bit to small to slide over the end, I did not attempt to force it on so I didn't bend the flash hider but is there something I'm missing on these problems?

Pictures of what you have would probably help. Yours has a set screw? I've only seen dust covers with a spring-loaded plunger, but maybe PTR has these made for their models? If it's a set screw, I would expect it to be removed completely before sliding the dust cover forward and off. As far as the bayonet ring and muzzle device, if they're both made to 22mm NATO specs they should work. Again, I'm unsure what PTR uses for muzzle devices these days.

The bayonet adapters I've seen are steel and are retained by a small machine screw and specially-shaped washer. There may be more than one design of bayonet socket adapter out there though. I believe I once saw a pic of a bayonet which simply inserts into an open cocking tube (with the dust cover removed). Of course it had a larger diameter than one that would plug into an adapter.

the FS may not be an actual HK91 FS.  I have seen a few for sale and some adds say "bayonet lug "AND" HK FS"

Maybe you need to swap out the muzzle device as well.??? As far as the dust cap removal, you may just have to work at it harder.  I had a CETME about 12 yrs ago and I removed the cap and installed a CETME bayo lug and it took some doing to get the cap off.  I had to swap out the muzzle device as well.

I am going a different route all together on my PRT91.  I still have my CETME bayonet just no longer the CETME so I just ordered a CETME bayo lug and I am hoping it will fit which it should since the PTR is made from HK tooling???

There is one more thing...I don't yet have the PTR91 either.  It will be a few more weeks till it arrives.  But it never hurts to plan ahead

I agree I started buying magazines 2 months in advance of getting my PTR. And got the bayonet end cap/dust cover thing.

When I replaced the dust cover with the bayonet adapter, I had to use pliers to remove the plug.  I held the plunger in with a drift pin punch and twisted the plug counter clockwise (like I was loosening a nut on a bolt) until the plunger was past the hole in the triple tree.  Once the punch was no longer needed, I kept turning the plug counter clockwise until it was backed all the way out.

I looked at the plug and the inside of the triple tree and could see no trace of threads, but it came out like it had normal (right hand) threads.  It was a fairly tight fit.  I then found that the threaded hole in the adapter does not quite line up with the hole in the triple tree.  In order to install the screw, I'm going to have to slightly enlarge the hole in the triple tree.

Oh yes a spring loaded plunger sounds more like what it is the dude in the video called it a set screw I thought. As far as what you said about the muzzle device possibly being not quite the right size that is kind of what I was afraid of. The whole bayonet mounting thing was really just something I wanted to do because I wasn't able to do so with my 16" AR and I always thought it was kind of cool. I don't know if I'm so committed to it to go as far as buying an HK muzzle device especially since I'd have to pay a gunsmith to swap the flash hiders. Thank you for your help, I will use my digital caliper to measure the diameter when I get home.

The cap that came on our PTR was aluminum and seemed to have been driven in by a jackhammer, I actually had to drill the top part out to get it to move, so yeah, seems that's the norm.

After I got it out, the bayo cap dropped right in and secures with a screw. Bought a HK bayonet from DSA, excellent used condition, works perfectly with the bayo cap, slides right over muzzle with minimal clearance, looks f'ing cool as hell.

Same rifle, yours should work the same way just fine.

I was told that the reason for the tight fit is because of the paint.  I ruined my front plug getting it out to put the bayonet plug in and inquired about it.  They told me it's the paint.

Update: I emailed PTR and they said that the powder coat finish does this. A representative of theirs told me to hammer it with a flat head screw driver to wedge between the end cap and the triple frame. I did this and gradually it came out. I had to hammer in the new end cap too. I'm not sure if it's supposed to be that tight but it seems to be working. Thank you all for your help.

ETA: PTR actually called me probably within 20 minutes of sending the email, that was fairly impressive.