One of my friends recently built an HK91 clone (his first attempt) on an FMP reciever. Functions great, trunions square, etc. It also shoots left, maybe 2-3 inches at 25 yds. His immediate fix was to mount an Eotech
He mentioned he might consider welding a bit more metal where the rear sight attaches to allow more rear sight travel adjustment, but I suspect he'll just leave the optic in place.
He finished his rifle in OD green duracoat. Very sharp rifle, and his welds were beautiful. His next project is an HK93/53 kit. I may have him build one for me...
My experience it based on the Century CETME. The way to fix this is to first heat up the triple frame (front sight) and see if you can turn it opposite from the direction you want your point of impact to move. Heating it up melts the silver solder.
It's nice to have a dot on the wall across the room. Look down the bore and center the dot in the bore. Then look at where your front sight is pointing through the rear sight. Does this confirm the point of impact being off? Do this before heating it up with a torch. Leave the rifle in the vice when you heat it up. A little bit goes along way.
If it will not turn you will need to remove the eye for the front sling mount and adjust sight and redrill for a new sling eye. Just curious, does the cocking tube have a hanger on the end for the handguard? If so, see how concentric the cocking tube is into the triple frame. There should be the same sized air gap around the cocking tube tip.
Sorry for not being real clear. Here's what I would do:
1. Field strip the rifle down to the barreled receiver.
2. Clamp the barrel in a vise and bore sight it using the dot on the wall method mentioned earlier. You will be heating up the triple frame where it attaches to the barrel to melt the silver solder. The vise must not be able to move so you can verify how much you rotated the sight.
3. I would then center the rear sight on it's platform.
4. If you have an Oxy-Acetelyene torch heat up the triple frame where the barrel goes through the bottom sleeve of the triple frame. Do not heat it to a cherry red, just enough to see if it can be moved. You just want to soften the solder so the frame can rotate.
5. Using a tapered Punch or a regular punch, as a lever, insert the punch into the eye of the sling loop and while heating the sight/barrel area try to rotate the triple frame opposite the direction you want your point of impact to move. This is definitley going to ruin the finish on the rifle in the area that is heated up. It will have to be refinished.
6. Check your sight alignment with the dot on the wall. A little rotation of the triple frame makes a considerable change of the POI.
7. Repeat 6 & 7 providing your sight is rotating.
8. If it will not rotate you will have to grind off the flaired end of the sling pin and drive the pin out. Then rotate the sight as necessary. Drill through the hole and install a new sling pin and flair the end of the pin.
Heres a site that goes into it a little better:
As I mentioned mine was a CETME and I'm not 100% sure that it will be the same but this worked for me. Good luck.