That's my experience, too. A rack grade gun often needs either a new barrel or at least a short cutback and recrown, for starters. My first Garand was a DCM Garand and didn't shoot all that well, maybe 3 moa as it arrived. After bedding it and other accuracy work I still had to cut about 1/4 inch off the muzzle and recrown it. The next group out of it after doing that was 10 rounds into 0.7 inches at 100 yards from prone position with sling, using handloads with Sierra 168's. It loosened up a bit over time and had metal fouling accumulation issues I've written about elsewhere. But it was pretty much a slightly sub-1 moa gun until I shot the barrel out. I just got a new Criterion barrel that's waiting for it in the basement workshop, and am going to be interested to learn what it will be able to do with that.
The accuracy work disqualifies that rifle for as-issued Garand matches, like the John C. Garand match. For those you are allowed to select parts and do a little limited fitting that doesn't require non-GI type parts. These include stock inletting in place of bedding, peening the gas cylinder spline to remove rotational cylinder fit slop, picking a gas cylinder lock whose threads happen to be timed to come tight right at 6:00, and etc.
This outfit claims in can get you a 1 moa rifle by careful selecting and assembly, but in my view the price takes away from one of the original purposes of the as-issued matches, which was to let folks as-issued guns have a match where they could be competitive without investing a fortune in bells and whistles.
The primer issue is worth attention. Slamfire's told us before of two dangerous out-of-battery slamfires he experienced with the Federal primers in Garands. I shot them for fifteen years in blissful ignorance and never had a problem. But I've tried to estimate, based on the number of slamfires I've been present for in the past, how common they are, and came up with something like 1 in 20,000 rounds on average. The ones I've seen have always been at big matches where the total number of rounds fired was about that many. 20,000 rounds, if fired through one gun, is enough to shoot out five barrels. With many competitors taking an interest in the game only for awhile, then retiring without having shot out a single barrel, you can guess that most never fire enough to have an even chance of experiencing a slamfire. So, anecdotal statements like "they always worked fine for me" can't be given much serious weight, as that's the probable outcome for most shooters, given their lifetime round count.
The out-of-battery slamfires tend to destroy the gun and injure shooters, so my policy is expect the best, but plan for the worst. The chances are that I could keep shooting the Federals successfully, but if I ever do have an out-of-battery slamfire that seriously injures me or the guy shooting next to me, I'll be questioning why I didn't take the simple precaution of using a harder primer. So, I switched first from the more sensitive Federal primers to the CCI to #34's, then just recently to the less expensive and more consistent KVB762 primer sold by TulAmmo.
The CCI's may be better for spherical propellants, as CCI optimized their magnum primers for those in 1989, but I use stick powders, so I can't say for sure. The Tula primers (they are made in Tula, Russia) are harder to seat, so I find I am running my brass through crimp removal even when there is no crimp, as that makes seating easier.
The 760 powder you have is too slow for a Garand. The muzzle pressure it presents to the gas system will be about 10% too high, and that can bend op-rods that aren't in tip-top new condition.
The Garand's sensitivity to charge weight and powders is a confusing topic. The Hornady loads, for example, are milder than loads used in the past or the surplus ammo you get today. If you look at John Clarke's match loads from the 1980's, Hornady will usually max out a grain or two lower. I believe this is in a misguided attempt to protect the op-rod, as the receiver and bolt and barrel of the Garand are extremely strong. I say misguided, because I believe the reduced loads are based on the assumption the lower the load the easier it will be on the op-rod. In fact, pressure in the gas system peaks about where Hornady's maximum loads are, then starts to go down as you load still higher. This is happens for several interacting reasons. One is that peak pressure increases faster than muzzle pressure as you increase powder charge. A higher peak puts more acceleration into the bullet earlier in its barrel travel, and one result is the pressure gradient between the bullet base and the chamber increases as the expanding gas has a harder time keeping up with a faster moving bullet. The faster bullet also spends less time between the gas port and the muzzle, giving the lower muzzle pressure less time to drive gas into the gas cylinder. So it works out, in some examples I've run, that as powder charge goes up the last three to five percent, the muzzle pressure might increase 6% while velocity causes gas port pressure exposure time to drop by 7%. The result is less gas driven into the cylinder rather than more. At the CMP forums, member Ericc, who has a gas cylinder pressure measuring rig, has posted a plot of gas port pressure profiles and impulse declining with charge increase above a point. The powder that showed this most clearly was the relatively fast Benchmark.
It's a funny business and the powders have to be in the right burn rate range for it all to work out, but it does. With very light bullets (100, 110, and 125 grain bullets) you'll find some newer powders like Reloader 10X and Benchmark expand the selection below the 3031 burn rate line. With very heavy bullets you can run H380 (civilian WC852) and IMR4320, but they will run the op-rod smartly, and there is usually no need to resort to them when slightly faster powders will be gentler with it and will shoot well. It seems to me John Clarke even had an IMR4350 load using 190 grain bullets. Heavy bullets give the slower powders more time to burn up and cause you to use lower charge weights of them, and that combination reduces port pressure. But it also means a slower bullet and longer gas port exposure time, so it isn't always a gimmie that it works out. The heavier bullets also increase the recoil impulse, and that pounds on the inletting and just generally shakes things around harder. Going to 190 just doesn't seem necessary when the 175 grain Sierras work so well and are so close to the weight of the 173 grain BT FMJ's the military qualified the gun for shooting in the first place.
For the 150's, I am gradually concluding the best powder may be the (unfortunately) expensive Vihtavuori N135. It produces lower gas port pressure and has low energy density, so there it has lot of bulk. You can use 90% load density under a Hornady 150 while running the gas port at 10% lower pressure than an 82% load density of IMR4895 does in achieving the same velocity. I'm not the only one at the CMP forums to have noticed the happy relationship between this powder and the 150's in the .30-06 and the low extreme spreads and reduced charge position sensitivity the high fill percentage produces.
The bulk 147 grain bullets I've tried have all been disappointing. This can change. One of the worst I ever tried were Winchester 147 grain bulk FMJ's; even worse than surplus pull-downs, never grouping under 3 moa no matter what you did with them. Then more recently I saw one post from a fellow who was doing alright with them. I think outsourcing has caused some seriously unfortunate quality irregularities in many bulk bullets.
Hornady still rolls their own and has the best cost/precision ratio in .308 150 FMJ's, IMHO. Sierras and Lapua's shoot great, but the price is significantly higher. If you're willing to spend money on high end bullets, the most accurate 150 I've found is the Berger flat base HP match bullet. It's a great bullet for learning the 150 grain accuracy limits of your rifle, but I don't want to spend .50 cents a bullet rolling ammo that won't qualify under rules at some matches. If I ever discover one of the smaller makers can give me the same precision as Hornady at a better price, I'll switch, but so far no luck.