My apologies for the tardy response, simply been busy elsewhere. Thank you for the informative and educational posts. I am considering this barrel replacement as a last resort in efforts to diagnose what I feel is poor accuracy.
The rifle (.243 cal) was acquired used in 1974/75. I would guess I have fired no more than 500 rounds through it over that 38 year period...probably 200 of those this year. In general, I enjoy the challenge of reloading and bench shooting to find the best accuracy possible out of a particular rifle. I have a shooting/reloading partner of like mind with over 30 years experience, so I am comfortable our reloads are as good and safe as we can make them. We are both rather meticulous in this sort of thing.
I feel this rifle should be capable of grouping 3-5 shots around an inch at 100yd. I could be wrong but that just doesn't seem unreasonable. Since February, we have worked with about eight different bullets and six different powder combinations and overall lengths (sorry don‘t have my loading data with me at present). Briefly, the rifle has always shot lighter bullets better than heavy, whether factory or reloads. But it is frustratingly inconsistent. Say a group may measure .5 inch.....and an hour later group 2-3 inch with the exact cartridge. The groups continue to expand the heavier the bullet.
We have tried all the tips we could find on line and here on the forum for possible solutions, but for brevity sake I won’t list them. If you are like me, I would question shooter ability and conditions, but both of us have shot it with similar inconsistent results. The rifle currently carries a new 6x18x44 scope, free-floated barrel, NOT bedded, with a 2.5# trigger pull. The bench is concrete and rock solid facing a tree lined shooting lane out to 350yd....the bench literally 5 steps from the reloading area. I’ve never seen a better set up.
One of my biggest concerns is not knowing the rifles history for the first 4-5 years of its life. It came with reloading dies, so abuse is a consideration....like pushing a bunch of 60 gr bullets at 4000fps. The current plan is to find a competent smith that has a bore scope and knows how to use it. If the original barrel is shot out, I will probably replace the barrel with the on hand spare although I hate messing with originality. But I agree with the overused phrase “only accurate rifles are interesting”. I know the rifle is not a bench gun, but feel it should be more consistent.....I could be wrong.
Sorry for the long winded response, but it’s a long story. And again, thanks for your time and comments.
Are the barrels of a 1970 M77R and a 1978 M77R of the same caliber and length, interchangeable? I feel quite certain they are, just thought it would be prudent to check here first for confirmation. 
