Larry, with the advent of modern barrels, the gap between semi-auto and bolt gun accuracy has decreased. I have two Rock River Arms AR Rifles with a less than 1MOA guarantee and both are under .75MOA accuracy with my choice ammunition and reloads. I know of no 26 inch threaded barrel models. I have several thumb hole stocked rifles and they are no less sturdy than the normal stocks and are slightly more comfortable when shooting. Fiberglass bedding on my Savage 110 Varmint did shave off .21MOA to .59MOA so it made a huge difference. Cheers and stay safe! You have an interesting project seems like.

I think why some people believe bolt action to be more acurate is because the whole process of cycling the action manually just kind slows you down and let's you focus in on the task, its a process many shooters learn when we 1st learn to shoot, so while a bolt action may not be more acurate ( or ate leats minimally) the process can help some shooters to make better more focused shots, one can train to do the same with a semi auto just most dont.