I had a chance for another round of testing over the weekend to verify that the modifications to the Catco scope rail (to correct massive elevation adjustment error) were effective.... Basically I had to clearance the rear mount and silver solder in a 1/16' shim to lower the rear of the rail. I've only found one other photo of that particular mount and that one had the front elevated a similar amount to what I took out of the rear. I think elevation problems were standard issue with the Catco rail. 

Anyway, the mod worked fine, and shooting from a bench and bags at 50 yd to mimic what I was able to accomplish when the elevation was almost too far off to get on paper, I now have a real idea of how the rifle is shooting after all the other changes. This is a rifle that would shoot 8"+ groups at 100 yd when we received it....

Now the rifle is shooting roughly every other (yep!) shot into 1" at 50 yards, with most shots touching. BUT every other shot is consistently ~1" high or low... So the pattern seems to go "good shot - Flier - good shot - flier - good shot - flier"... I've never seen anything quite like it and am curious if anyone has any thoughts on where to look for the inconsistency. In my opinion something is moving and getting hung up on the way back to its natural home.... but what?

The obvious thing would be movement in the scope mount but I'm checked and double checked and can't find anything. Once I clean up and re-blue it I'll bed it to the rifle with epoxy for good measure but it seems to be tight and right....

Here's where the oddysey has led me so far. 

1) recrown Barrel

2) clearance ati synthetic forend near nocks form (turns out is was a No. 1 Mk 4 stock) to match barrel diameter and allow proper seating.

3) epoxy bed stock and shim bottom to best approximate factory fitment.

4) polish and adjust magazine feed ramps, guide "lips", and latch. Block mag for hunting capacity requirement.

5) free float barrel in forend (There's nothing to replicate OE bedding anyway. Seems best choice)

6) remove OEM iron sight bases to reduce weight, simplify harmonics

7) modify aftermarket Catco scope rail to correct elevation to be within scope adjustment range.

Next steps to chase down the flying gremlin will be to verify free floating of barrel, maybe add a rudimentary barrel tuner (IE rubber buffer) to play with harmonics, double check scope rail for movement, and possibly thread forend screw hole (where OEM mid band used to live), to add an adjustable pressure pad and dampen harmonics.  

Any other suggestions? 

Thanks all!

What is the lateral extreme spread of your groups, this might tell you the potential of the barrel. 

I did a LOT of fitting work on the forend (and sorry, I probably did misstate the model # that was printed in the stock.) First time I took the rifle down it was literally pivoting around the action screw with no contact at the draws and the barrel pulled down into the forend with the barrel band... it was a hot mess for sure. I have a vague memory reading that there wasn't a 2A1 specific ATI option so people made due with another model that was a "close fit". But that could be wrong and doesn't really matter. I'm just running with what I have. The forend is now a very tight tap on and tap off fit and makes good, even contact at all the critical points. I hadn't considered the buttstock though. That feels tight but I never did remove it. Might be worth a look.

If it's not something loose then it almost has to be something catching and sticking out of place instead of returning to its natural home. I just don't know where to look. Bedding the barrel would be tough because of the stock design (would take a pint of Epoxy anyway). But I could manage a pressure pad. Anyone ever tried that on a sporter?

Thanks again folks!