Here's the short version Tony.
We have all seen claims about moly coated bullets and perhaps a few horror stories from shooters who tried them. To a some extent most of the positive claims are true for "certain" shooting applications. The horror stories about corrosion and wrecked barrels may also be true depending on how these products were used.
Bench rest shooters are usually the first to adapt new technology and the first to discard things that do not work. A few years ago some benchrest shooters where shooting moly bullets. Few competitive bench rest shooters are shooting moly bullets (although a few may still be adding moly to their powder). This does not mean moly bullets have no valid applications...just that the benefits are not applicable to all forms of shooting.
Increased Accuracy
An early claim regarding moly coated bullets was increased accuracy. It was surmised that less bore friction would reduce tearing of "fins" on the bullet's base producing less turbulence in flight. To our knowledge this claim has never been substantiated. Even if true, reduced turbulence will only be evident at extreme long ranges. We expect the improved accuracy some shooters see are due to other factors such as moly's affect on the pressure curve, neck tension, etc.
Moly Will Change Your Loads
Switching to moly will change your load performance. Velocity and pressures should decrease when switching to moly coated bullets. This is not bad if you have a chronograph or the time to work up new loads. Because bullet to barrel friction is drastically reduced you can often push bullets faster and still stay within safe pressure limits. Normal loads that crater, flatten or puncture primers "may not" when shooting moly bullets. The increased velocity that "might" be achieved with moly coated bullets is often sufficient to reach a higher velocity "sweet spot" where groups will tighten again, or allow you to fire heavier bullets at acceptable velocities.
Note: The bearing surface of a bullet increases exponentially with bore diameter. Generally the larger the caliber, the greater the difference in acheived velocity for a given powder charge. Smaller/lower pressure calibers often show only 10 to 20 fps velocity reduction while large magnum calibers may show 100 to 150 fps velocity reduction when switching to moly coated bullets.
This also may introduce another issue. As bore friction is reduced, the bullet may accelerate down the bore as if it has lighter mass. This can result in secondary pressure spikes if the powder is barely fast enough for the bullet's rate of acceleration.
Reduced Throat Erosion
It was originally claimed that throat erosion was reduced by using moly bullets. The jury is still out on this one...but most shooters tend to discount this claim. Throat erosion is primarily caused by hot gasses entering the throat behind the bullet, no doubt if the only difference is a moly coated bullet, throat erosion might be reduced because gas pressure and heat is reduced. But most shooters will add powder to regain the velocity lost when switching to moly. This should then produce similar pressure, heat and gas erosion of the throat.
As just one example of the bad information that has been disseminated about Moly, on page 231 of a popular book about long range accuracy the author states "The drop in pressure and velocity is NOT caused by a reduction in barrel friction as proposed by Norma and others. It is caused by the hot propellant gasses vaporizing the coating resulting in a cooling of about 400 degrees F." The author says he is "certain" about this because of "sophisticated internal ballistics code"....QuickLoad?
I understand the author's son is a chemist and that may have contributed to a "myopic" conclusion. If they had access to good pressure testing equipment we doubt they fully grasped the limitations of the mathematical algorithms used for the analysis or all of the issues affecting pressure. Increase pressure on a gas and its temperature increases, reduce pressure in the chamber and the temperature will be lower. His own tests indicated pressures dropped from 54,000 psi to 47,000! This amount of pressure change can be produced with bare copper bullets by changing seating depth and/or neck tension in most calibers.
Supposed proof is when moly was introduced to powder without coating the bullet, pressures and velocities were still lower! Interestingly no one seems to be able to duplicate this experiment with a reasonable amount of moly (micro grains) and a cold clean uncontaminated barrel. Any detectable reduction in pressure is easily explained by the moly working as a slight burn rate deterrent as would be expected when any inert substance is used in sufficient quantity to coat the powder granules.
Tests conducted with good pressure instrumentation shows it is indeed moly's ability to reduce friction in the cartridge neck, throat and bore that reduces pressure. Norma was correct!
Less Copper Fouling
Perhaps the most legitimate claim about shooting moly is reduced copper fouling but even this claim cannot be universally applied. In quality barrels, copper fouling is often minimal so switching to moly may offer little advantage; especially if you are not inclined to put large numbers of rounds through the barrel before a thorough cleaning. This is what benchresters discovered. A good barrel will easily shoot 10 rounds without copper fouling. One characteristic of moly is that when starting with a cold clean bore it may take 10 to 20 shots for the bore to coat with moly so velocities stabilize. Benchresters concluded when firing only a few rounds the introduction of another variable is a bigger disadvantage than the benefits of using moly coated bullets. If you shoot 50 to 100 rounds at a time and don't mind 10 fouling shots before velocities stabilize you might still consider using moly coated bullets.
Moly will build up in any barrel and is more difficult to remove (see below). If your barrel is prone to copper fouling, moly may help extend the number of shots before groups start opening up. However, if you can shoot 25 rounds of bare copper bullets without group sizes increasing there may not be a significant improvement by switching to moly. The bottom line is we have seen barrels that seemed to like moly, some where there was no appreciable difference and some that flat out did not seem to like the stuff.
Less Barrel Heating
I believe a primary benefit to shooting moly coated bullets is one rarely claimed by those selling moly products. In rough barrels that "walk" as they heat, moly seems to provide significant advantage when firing quick repetitive shots. I have a rifle chambered in .223 that is sensitive to heating even though it has a relatively thick production varmint barrel and has been well bedded. I used a thermometer to compare barrel heating and it took 14 moly coated rounds to heat the barrel to the same temperature as 5 copper jacketed rounds. The barrel does not copper foul more than most production barrels, but it is still rougher than I would like. In this rifle group sizes definitely benefit from moly.
Note: The bearing surface of a bullet increases exponentially with bore diameter. The larger the caliber the greater the friction reduction benfit. Smaller calibers often show only slightly less heating when shooting moly bullets. Smooth small caliber barrels may show no benefit at all. The above mentioned rifle has since been fire lapped so it no longer heats as badly. I compared heating again with bare and moly bullets and found only a 10 degree advantage after 15 shots of each...hardly enough to make a difference.
Cleaning Moly
Serious barrel damage can result from moly buildup (caking). All barrels will "cake" with moly but a barrel prone to copper fouling will collect more moly at fewer round intervals. Moly usually builds up "rings" at certain points in the bore. If the build up becomes sufficiently severe bullets fired through the restriction will expand the bore at these ring points. This is why some highpower shooters have wrecked barrels after more than 100 rounds.
A traditional measure of barrel smoothness and acceptable copper fouling is the ability to shoot at least 10 to 15 shots before a patch wet with Sweets turns green from jacket material embedded in the bore. If after a single shot there is indication of copper fouling, you should break in the barrel before shooting moly bullets. When copper fouling is minimal your bore should resist excessive moly caking but it will still need to be properly cleaned. MOLY IS NOT A WAY TO AVOID CLEANING YOUR RIFLE. (For more info. On the subject of copper fouling and shooting in barrels click here.)
I have tried several moly cleaning methods and settled on the one recommended by Walt Berger. Shooter's Choice, Hoppe's No. 9 or hot water and dishwasher detergent gel as some suggest simply does not work and may actually promote corrosion. Berger recommends using Kano Kroil penetrating oil and USP Bore Paste (similar to JB) at least every 80 to 100 shots (shorter intervals in guns prone to copper fouling). Their instructions are to run two patches wet with Kroil through the bore, a dry patch, a patch of USP Bore Paste, two patches of Kroil and a final dry patch. When running the bore paste through the bore they also suggest short 2 inch strokes. This method seems to clean caking out of the gun and smoothes the bore nicely without removing all the moly. You can purchase Kroil directly from the manufacturer at (615) 833-4101. USP's phone number is (412) 621-2130.
If you have a rifle that can shoot 25 rounds before is becomes copper fouled it may be easier to remove the copper with conventional solutions then trying to remove moly using the Berger prescribed method.
Corrosion
We have not seen any corrosion problems when using moly but we also do not store rifles without cleaning them thoroughly. There have been numerous reports of M1 shooters who failed to clean their gas tubes after shooting moly bullets only to discover nasty rust developing. If you shoot moly, make sure you clean every part of a semi auto's mechanism where powder and bore residue can collect. Most moly products do contain trace sulfides which can become acidic and promote corrosion in humid environs.
THE BOTTOM LINE
It is safe to discard most of the "hype" regarding extended barrel life, ease of cleaning, etc. and focus instead on performance. Nearly all competitive shooters have discarded Moly as a bad idea that just introduced another variable to control. Copper is not difficult to remove with proper solvents and cleaning tools. If you see no substantial accuracy improvement with moly bullets then you may have no reason to use them.
On the other hand if you have a barrel that simply will not shoot well no matter what you try; or it "walks" badly as it heats, then moly coated bullets may be worth a try. Some rifles seem to prefer moly, others show no substantial difference. We have concluded there is a direct relationship between the quality of a barrel and the benefits accrued from using moly. Generally the better the barrel, the fewer benefits. Good barrels foul less, do not change shape as they heat and are easy to clean.
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