I have seen some studies that just about say that moly will ruin your barrel... attracts water, causes corrosion, etc. This has never been my experience.
Moly works great in some situations, and has much less effect in others.
Works Great:
- in smooth clean barrels
- reduces copper fouling
- with moly-coated bullets in pre-molied barrels
Much less effect:
- in barrels with a rougher internal finish
- when the shooter tries to coat the bore by shooting moly-coated bullets
- when coated over a copper-fouled bore
Many people who have tried moly have done so in such a way that almost guarantees problems.
A rough bore that fouls heavily will not be fixed by using moly. You'd have to fix the bore (lap it) before fouling problems will abate.
Expecting the bore to be evenly and quickly coated with moly by shooting coated bullets is wishful thinking, in my opinion. Reportedly, it takes 25-30 rounds before enough moly is in the bore to be effective. During this time the same bullets are causing copper fouling as they travel down the bore after the moly wears off. During this time the friction qualities of the bore are changing, making consistent shooting very difficult.
Moly coating over copper fouling is not a good thing. Copper fouling alone is bad enough... Moly over the top does not help.
Here's my moly routine:
I carefully clean the bore until there are no signs of copper fouling. If I'm cleaning a bore that I have used moly in already, this job is much easier for me.
When the bore is clean and dry, I take a dry patch and put dry moly powder on it. I run this patch through the bore several times to burnish the moly into the bore.
After the bore has moly burnished into it for its full length, I run one dry, bare patch down the bore to get any loose moly out.
The rifle is ready to shoot. The 1st shot from a cold, clean barrel is at the same point of impact as further shots.
Cleaning:
Use a high quality, one-piece cleaning rod. Use the segmented, screw together rods for emergency use ONLY, or (better yet!) toss them.
Use a rod guide if at all possible.
If it is a custom barrel, then it has probably been hand lapped and has a very smooth interior finish. These barrels foul very little, especially with moly. I don't use copper brushes in them; nylon is plenty.
I generally avoid cleaning solutions with high ammonia contents. Butch's Bore Shine, Shooters Choice, and TM Solution work well.
USP Bore Paste or Iosso paste work well when I occasionally want to clean down to bare metal, remove the moly and start over.
In very high-quality, custom barrels I have great results with Kroil, USP Bore paste, nylon brushes, and patches only. I only use the USP Bore paste after a couple hundred rounds. Regular cleaning is with Kroil, the brush, and patches.
Using moly in an AR-15 may not help much. Chrome-lined bores are intended to foul less and hold up better under heavy use, so you'll see less benefit from moly.
Switching back & forth between moly and non-moly bullets will not ruin anything, though it is not optimal. I do this occasionally... though I am extra careful to clean well after shooting non-moly bullets.