This morning I saw a BM-59 built with a M14 receiver- how ?

I have a rifle and pistol range on my farm. Milk jugs set up full of ballistics gel, all built into a hill with a concrete wall(I scrounged the blocks off Facebook classifieds) and then sand bags behind that. You can walk across my creek bridge and shoot up to 250 yds.

I heard gun fire this morning, and it sounded a little too close to the house. I figured I'd better go check. I got on my Satoh and road down the hill and and heard a rifle report and I yelled cease fire. My guts are still hurting from the 20 cm, .835kg gallbladder removed a week ago, and I probably seemed more pissed than I was on account of that. The shooter ceased fire.

A man in his 40's was shooting at targets from my 100yd marker, he'd put the targets on my dedicated target hanger Toyota MR2 hood, and was nicely not destroying my ballistics gel.

I have no trespassing signs all around the farm to cover my butt, in case someone gets hurt poaching or planting cannabis.

He seemed awfully in a hurry to get off the property- he'd driven an older K5 Blazer .58 miles off the paved road, past a gate and fence, numerous signs, and past a tree line that hides the make-shift range.

I noticed the rifle he was shooting was an M-14, but when I got closer I realized it was a BM-59. I told him it was a nice rifle and he thanked me, told me it was built off a Fed Ord M14 receiver, that he couldn't stand ruining a Garand, I tried to tell him to stick around, but I keep a commercial paratrooper .30 carbine in a lined sheath on the side of the tractor hood, and I don't know if he thought I'd call the cops on him or shoot him.

But... He split, and I was left with the unanswered question:

How can a BM-59 be built with an M-14 receiver ? The part of me that owns the lathes, surface grinders, horizontal and vertical mills- says much machining.

But can anyone fill me in on what machining was necessary to use a commercial M14 receiver with a BM59 parts to create a BM59 hybrid.