I recently de-coppered the barrel of my old .243 Rem M700 using Montana Extreme. Fired a 3 shot fouling group with Winchester Deer Season 95 gr loads that printed 5/8 of an inch. Then 95gr gr partitions in a handload over 41gr of H4350 that “grouped” at 3 to 4 inches. What the hell???

Your rifle prefers the factory ammo?

It did today. Go figure.

Put the gas to that Partition load.

On occasion Partitions like to be pushed hard to get top accuracy.

Did you check distance to lands between the two bullets?

And yes, I understand the NBT’s are factory. Pull one of the NBT’s and use a fired case from the same box of ammo and check.

Hell, I'd be happy with 5/8 inch 3 shot group with cheap factory ammo. Winchester Deer Season is available everywhere. Grab a few box's and go hunting.

Maybe your hand loads just suck

Was the handload something you previously worked up, or something you just threw together? Your OP is a little vague. Thread title talks about "decoppered 243", and then you state your rifle shot a 5/8" group. No info to go off of as far as I can tell.. You are either telling us you shot 1 3 shot group from the factory ammo, then moved on to the handloads. There's not statistical basis to go off of there. Shoot some 5 shot groups with the factory ammo. If you are still shooting 5/8", your rifle is good to go. You should be able to best any factory made ammo, with a tailor made handload. That is if your rifle likes the bullet you are using.

I could never get the 95 gr Partitions to shoot worth a darn in my .243 Win 788. 100 gr. and 85 gr. are sub-MOA. 95 gr. sucked

Thanks. Interesting - I have a box of the 85 gr Partitions I haven’t tried yet. Will also experiment a bit with seating depth and powder charge. Was worried I might have exposed a very worn throat when I did the thorough decoppering, but then odd it was happy with factory ammo.

^^^^ This. 41 grains of H4350 is a pretty light load.

Powder charge is where I would start.

Some room above 41 grains in my opinion.

When you luck into something your gun likes, go with it. Sometimes there is no reason why a gun prefers one thing over another.
But before you give up on the partitions, give it a little more gas, like others suggested.

If it previously grouped ok with the partition load, your barrel may need a few more fouling shots to settle down and like them again. Might be worth wasting 20 or so partitions to find out.