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Posted: 4/26/2025 9:53:28 AM EDT
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Curious to hear from other people with experience with the M249s, have you noticed rust or oxidation on any of the welds? I noticed what appears to be some kind of oxidation on the seams of the weld that runs along the front of the magwell on mine. The color seems different from typical rust, and there was no rust-colored residue when I wiped it with an oil soaked wipe. This gun has not been abused, quite the opposite. As a matter of fact, it hasn’t been fired yet and has been stored still in the oil from the factory, plus more that I added, in a VCI bag. I am wondering if this is normal for the M249s to have these marks? Did I get a lemon? If you want to see a picture, I posted one on FN Forum, here: https://www.fnforum.net/threads/rust-on-m249s-magwell-weld.234029/ |
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I don't have an M249 to share observations, but it is most likely oxidation from one or both of two sources. Oxidation slowly compromising the surface finish layer on top of it due to surface prep before finishing, slight amounts of surface oxidation that was remaining in those areas. Second possible cause, especially the larger, browner spot could be a small mass of solid oxides that were not fully fluxed off during welding and settled into that edge of the weld; with the high heat creating a more solid mass that is does not have dust or flaky layers like slow surface rust progress creates, so it's less likely to wipe away as you report. Good news is that's all located at the edges which are providing almost none of the strength of the weld. (I've seen the same thing around the cross block welds in HK 9X/MP5 trigger pack frames.) FN and DoD would not consider this a problem since normal use is going to completely wear the finish off many areas of the receiver, and it will thereafter depend on proper cleaning and continuous lubrication to avoid the progress of corrosion in those exposed areas. That is to say, this is a "safe queen" problem, by which I mean no insult to anyone wanting to preserve any firearm in the most pristine condition possible out of interest in collecting, preservation, investment, etc. If this was some piece of machinery where any sign of corrosion was unacceptable, or undergoing a full rebuild to reset the lifespan to as new, you would clean off all surface finish in the area, grind the affected areas out to clean steel, and reweld, liquefying all metal adjacent to the grind, and then strip and refinish the entire assembly. Obviously that's a little too dramatic to justify for a valuable firearm, especially when there is no strength compromise, and I would never reheat welds on lighter sheet metal assemblies without all the parts locked into the same jig that was no doubt used by the factory to keep all pieces in alignment during welding and cooling. Moisture and oxygen are the two things in normal storage that accelerate the progress of iron/steel corrosion. Keeping it well oiled is the easiest way to reduce exposure of those areas to both. If you want to do more, you could find a clear coat finish that is oil resistant. Clean and degrease those areas with an appropriate solvent (or even just a few drops of Dawn in a small amount of warm water), scrubbing those areas with the narrow end nylon brush from a standard M16 cleaning kit to avoid damaging the finish. Dry with paper towel, then dry with a heat gun in an area or during weather conditions that are low humidity, then hit it immediately with the clear coat. Using a small paintbrush (like model painting) I'd brush the clear coat just over the affected areas extending enough to seal them off from air exposure. Most clear coat is found in spray form so I would spray a puddle onto a clean surface and then dab it out of the puddle with your brush. Leave the clear coat exposed to the air long enough to cure based on the label, and maybe give it an additional 24 hours since your recoating of oil is going to prevent further air-curing. Then recoat the whole area with a good preservative oil coating. That's about the most extreme steps you can take, which is just minimizing oxygen and humidity exposure, without removing or altering the rest of the original finish. If you live in a coastal area, I'd be more likely to go to the trouble of clear coating as I have seen the salt air really accelerate corrosion in other types of heavier machinery. |
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Sir, thank you. That was very helpful and worlds beyond my expertise. It’s pretty damp most of the year where I live so looks like clear coating may be the way to go. It’s a bit of a bummer to have that happen to an expensive rifle that is, as your rightfully pointed out, a safe queen. Especially since I thought I was doing all the right stuff: extra preservative oil, VCI bag. As they say, “first world problems.” Thanks again for that, super helpful. |
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Originally Posted By MJ11B4P: It looks like dried preservative (Cosmoline) residue in that hi-res photo. That what I was hoping, but it didn’t come off which scrubbing with a towel, and when I looked closer I could actually see the edge where the finish had broken away, eesh  . From my internet search, I read that “straw color” comes from light oxidation during the welding process. (But I am certainly no expert.)
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I can easily see where this would be distressing to see on a 10k dollar purchase (particularly one purchased as an investment), but as someone who carried one for a living, the idea of that small speckling of off-coloration causing a panic is... well, amusing to me. Small piece of advice to you, if you find yourself in a panic over anything on that gun, just go carry it for 12+ miles and see if you still want to baby it or if you'd rather chuck it over a damn cliff 
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Originally Posted By BigBurkeyBoy: I can easily see where this would be distressing to see on a 10k dollar purchase (particularly one purchased as an investment), but as someone who carried one for a living, the idea of that small speckling of off-coloration causing a panic is... well, amusing to me. Small piece of advice to you, if you find yourself in a panic over anything on that gun, just go carry it for 12+ miles and see if you still want to baby it or if you'd rather chuck it over a damn cliff Exactly how I feel about the ‘240s in my NG unit. Luckily I’m not one of the bigger guys so don’t have to carry them very much. |
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