Centre-punch the site of the hole, with the receiver and barrel set up and "lined up" (sights etc) in a vice. Remember once you have drilled the hole, even the pilot hole, you will be committed...no more sight alignment adjustment after the drilling.
Drill using a 1/4 inch or 6,5mm drill. Then get a 7mm parallel reamer ( an adjustable one may be needed) and finish the hole to 7mm...assuming you have set everything up correctly,and drilled True ( ie 50% in the barrel, 50% in the wall of the receiver, and come out horizontally "level". All this requires a professional Drill press, or better still a Milling Machine or a heavy Mill-drill combo...NONE of your $99 Walmart specials....
Once the hole is cross drilled ( by setting up to drill Vertically in the Milling Vice) and reamed, you can prepare the Locking Pin. It needs to be very slightly tapered ,with a "rounded" lead edge. The Back edge can be flat.
DO NOT Hammer the PIN; lube it lightly, then Tap it in to start it. Check that it is square to the receiver with an engineer's square...Then Move the whole lot to a 5 ton Arbour or Hydraulic press, and Press--Repeat--Press the Pin in, using a flat face on the press to do the job. The Job itself should be kept square by locking up in a Milling Vice, firmly bolted to the press table. Use form-fitting clams if possible ("receiver Blocks")
That is the way they do it in the Factory, so you can't go better than copying the system.
AV Ballistics Film Ordnance Services
PS BTW NB
I am assuming above that the receiver is "unfinished" ( ie there is no hole in the trunnion part yet...)
If there is a 7mm hole already in the trunnion, but no corresponding slot in the barrel, proceed as follows:-
Try to get a 7mm End Mill ( four flutes--an "Unconventional" size), and use in a Mill Drill, using the receiver trunnion hole to "Guide" it true. The four cutter edges will cut the "half groove" in the barrel, and line it up with the existing holes in the Trunnion. Then clean up with the 7mm reamer. Everything must be "square" otherwise the cutter will "run out" and bugger your hole. Work the mill drill, advancing slowly...patience is a virtue here--and clear chips regularly, with either compressed air, or a squirt bottle of cutting fluid.
Replacing an existing barrel in a gun is easier than fitting a new barrel.