The best answer is to try it yourself.
That's not skirting the issue, you just can't get the same answer from two different folks concerning moly coating. Gale McMillan (God rest his soul) voided his rifle's warranties if he knew moly-coated bullets were used. I heard about the Sinclair issue. In the meantime, Walt Berger, of Berger Bullets and benchrest fame, still stands by the moly-coated bullet and sells them. He also posts an excellent technique for properly cleaning a barrel after using moly on
http://www.bergerbullets.com. George Vais chastised me on the phone (he's got a funny accent) for running moly through my Kreiger barrel. On the pro side,
http://www.xtremeaccuracy.com has a full page concerning the myths and truths of the technique. In Precision Shooting, I've read articles by separate authors either deriding or praising moly. Sierra and Hornady still sell moly-coated bullets, ostensibly for the business ($) aspect, but they were talked into selling them on more than a whim.
Here's what I do know, from my own personal experience, about moly, after 4 years of loading 10,000+ moly-coated rounds in 24 different calibers:
1. My velocities dropped, I had to bump up the loads to get back to uncoated bullet velocities, validates the lower friction/pressure phenomenon.
2. Copper fouling in my 6.5-06 disappeared, even after 88 round matches. This gun has a 25" Kreiger barrel, mirror smooth inside, but 120gr Nosler Ballistic Tips squirting out over 3200fps still shed some jacket copper near the muzzle. Sierra MatchKings were even worse. After moly-coating said bullets, nary a problem.
3. You don't get something for nothing. You're simply substituting copper or lead or brass fouling for moly fouling. However, unlike copper, lead, or brass, molybdenum disulfide is considered an industrial lubricant, which is not a bad thing to leave in a location of high friction, kinda like PTFE in engine oil additives. Now if they could come up with a PTFE coating that withstands the faster rifle velocities, woohoo!!!
4. Clean bores, either via new barrels or old dark Cordite-fed military bores having been polished with JB Bore Paste or Pol, need about 5 rounds of the moly-coated fodder to settle down. If you clean the bore afterwards back to clean metal/white patches, you have to lay down moly once again to get the groups to settle down if you intend to use moly further.
5. Throat erosion using moly bullets has decreased compared to "naked" bullets. I noticed this especially in my M14NM with medium weight match barrel, and the above 6.5-06. Dunno if it's the lower friction, or the moly conducting heat away from the leade, but it's a factor. It makes me happy, more rounds can go downrange and still group decently before I have to either rebarrel or set things back a thread and rechamber. Kreiger, Obermeyer, Rock, and Douglas premium barrels ain't cheap, so the moly is a bit of an investment.
6. Pitted, cratered, dark, or frosty bores from old warhorses and commercial arms, like my 7.62x45 Czech VZ-52, 8mm 1917 Amberg Gew98, .30-30 Savage 340, or .303 No5Mk1 Jungle Carbine, appreciate the paving over of the bore imperfections when moly-coated ammo is used. The point of impact shifted, but the copper fouling again decreased, and the groups didn't open up after 20 or 40 rounds as earlier when the potholes filled with copper. Cleanup afterwards was a good bit easier. On a couple of the warhorses, the grouping actually shrank some, I'm assuming a better-fitting bullet. On the autoloader VZ-52, the brass ejected to a different location, probably because the pressure curve had changed.
7. Accuracy in good barrels hasn't really changed much, if it shot well with naked bullets, it still shoots just as well with moly-coated fodder. It did prevent groups from opening up due to copper fouling.
8. Moly coating cast or swaged lead bullets didn't do a whole lot for leading or accuracy, at least in my tests with .38 Special and .45 ACP. Cleaning the leading was a bit easier, though.
9. Too much moly and the bullets look like hell, and leave excess powder on the feed ramps, chamber mouths, etc. I've seen some folks even experience misfires because there was a moly buildup so heavy in their chambers the rounds wouldn't seat properly. But that's a technique thing, you don't need to cake the stuff on, just a thin impact plating and wiping off any excess with a clean towel will do.
10. No increase in pitting or corrosion in either the smooth, shiny barrels or the old warhorses. And when you've got money sunk into premium barrels and nice guns, and you live in hot, humid, salty, East Coast Central Florida, you look for corrosion on a religious basis. In fact, my Remington 700PSS in .308 has seen nothing but moly-coated 168gr Sierra MatchKings since the day I bought it new.
11. In gas autoloaders with pistons, like my Bulgarian SLR-95 (AK clone), as well as my Garand, M14NM, BAR, and VZ-52, the moly does find it's way to the face of the gas pistons, and even the gas cylinder. So if you're insistent on cleaning up the moly afterwards, don't forget those parts.
I'm in the process of assembling a 6.5-300 Weatherby Magnum, on a Sako action, 1-8" twist barrel, for 155gr Sierra MatchKings. Will I moly coat those bullets? You betcha! Unless something better comes along to prevent me from chewing up a decent barrel in less than 1000 rounds.
Differential metals, with moisture of a proper pH or acid between them, do indeed constitute a battery via electrolysis. The rate of electrolysis depends on the strength of the acid, the two metals' conductivity, surface area, etc. However, I'd be no more worried about the battery etching effect of moly on a bore than I would a layer of lead or copper plating, aka fouling, which would also produce that effect by virtue of the chemistry involved.
The sulfur in the disulfide component of molybdenum disulfide is not as easily liberated as some folks would have one believe. If you leave or allow enough moisture in a bore for long enough, moly or not, you're gonna get pitting. Moly does not provide an excuse to just stop cleaning, especially in environments that are conducive to corrosion. But you don't need to remove the moly to inhibit corrosion, just clean the bore properly via the Kroil technique listed above at Walt Berger's site.
Or, if you try it and you decide it isn't for you, you can indeed clean out the moly and go back to naked bullets, no harm done. I'm not really a poster child for the technique, I just wanted to see what all the hype was about and ended up finding some real benefits. But my Cheerios still get soggy after a while in milk, the moly hasn't done a darned thing for them.
