I think I read somewhere that TMJ bullets create more copper fouling than FMJ. Is this true? If not, are there any downsides to TMJ?

There's no reason that the design of TMJ bullets would cause more fouling than other jacketed styles. A different jacket composition could cause it, but that would depend upon manufacturer and, possibly, specific bullet.

Something to keep in mind is that velocity does in fact affect jacket fouling: the higher the velocity, usually the more fouling. In the CCI Blazer and Speer Lawman lines of common autoloading pistol cartridges (9mm, 357 SIG, 40 S&W, 45 ACP), most of the higher velocity loads use TMJ bullets. Included are the 357 SIG 125 grain and 40 S&W 155 and 165 grain rounds. Because of their higher velocity, we would expect more jacket fouling.

In my experience 357 SIG ammunition fouls worse than common 9mm Parabellum loads, but I believe it's simply due to its higher velocity. I haven't noticed the same with the lighter 40 S&W bullets, but they don't reach 357 SIG speeds.

So with the American Eagle ammo, where the TMJ is actually slightly lower velocity, it should be at worst the same amount of fouling then, right?

I think what I had read was something about TMJ bullets being plated and not jacketed so they for some reason foul more..I dunno.

quote:

Originally posted by marlinspike:
I think what I had read was something about TMJ bullets being plated and not jacketed so they for some reason foul more..I dunno.

If the jacket material is different, that could affect the fouling. I haven't seen it, however, except as I noted about the 357 SIG ammunition.
On the other hand, my barrel cleaning method removes all fouling after every range session and minor differences may not be obvious to me.

I don't think that you can make any generalization about the degree of copper fouling between bullets with "Total Metal Jacket" vs bullets with "Full Metal Jacket". I agree with sigfreund that jacket composition has more to do with copper fouling regardless of whether the bullet is TMJ or FMJ. I've seen the term "TMJ" used to simply denote that the lead core is completely enclosed in copper. I think that there are some plated bullets out there that are referred to as TMJs even though they technically aren't "jacketed". I would expect plated bullets to foul more than jacketed bullets - especially at high velocities.

If you get down to the basics of the OP's question, it should make no difference. The difference between a FMJ and the TMJ is that the in the TMJ, the base of the bullet has copper jacketing over the lead. The part of the bullet that is copper jacketed and comes in contact with the barrel are the same between FMJ and TMJ.