I've got an old ACH ballistic helmet with a mount for my NVG. I've seen on Ebay ( maybe there are others, I don't know about) who can trim these style helmets into a high cut helmet. I bought this helmet years ago from another member here on the EE, and like the fact that it's ballistic rated. However, it's rather heavy. I assume trimming would cut some of the bulk.
1. What would be the downsides to trimming a helmet?
2. Would I be better off just selling this helmet and getting a cheap bump helmet to put my mount on?

1.  None.
2.  Why do you need ballistic protection?  If it's to protect your noggin from frag or impact, rock on.  If it's just to hold your NVG you can do a whole lot better, lighter, and cheaper with something as simple as an airsoft or hockey helmet.

The big downside to cutting a helmet is if you don't do it properly. You will need to inspect the cuts and make sure to re-seal and coat the edges. If you do not do that, water will seep into the cuts and begin to break apart the layers of aramid in the helmet.

As said, if you just want something to mount NODs to, there's better options.

I've done this, several times in fact. It doesn't shave off that much weight. I use a grinder with a cutting wheel. At the factory when these are made they use a rotary tool (die grinder) and a cutting wheel to cut the excess material off after the molding process.

I've also drilled holes in these helmets before. Use a small bit. Drill though to make a pilot hole. The step up to your correct bit and drill halfway on both sides to mitigate "tear out" same as you'd do in wood.

I don't recommend doing this if you need ballistic protection. And frankly the amount of work and nasty dust this creates ain't worth it.  You really need to wear a respirator while doing the work Buy an airsoft $50 bump helmet and sell your ACH if you just want something to mount your NVGs to.

Regarding resealing and coating the edges...pull the edge trim off a helmet and you'll see it's not sealed with anything. Just cut and sanded. The edge trim looks like they either glue it on or it's pressed on and heated.

I haven't cut a normal cut to high-cut, but I did drill out a DH132B for a harness conversion and to mount rails and a night vision mount, the above posts cover the process pretty well.

I will say this: I never wear it, it's too heavy and I'm not getting shot at, but I like having it. My Crye Skullcap only comes out when I'm traveling and have limited space to pack.

What gets the vast majority of wear is a $50 dial suspension airsoft helmet with $100 4D pads.

Send it off to KustomACH.com

Dude does a lot of work for a very cheap price. He will help you cut it, reseal it, repaint it, etc. all for a very reasonable price.

I've got one sitting in the closet and hope to never be so desperate as to have to don ever again

This one is total nonsense fuddlore.

OP, the downsides are decreased protection coverage, "if you fuck it up, you have a fucked-up shell", and the health risks of inhaling Kevlar dust if you do it without a respirator.

If you want a high-cut helmet just for NODs/comms mounting but you don't need ballistic protection, you can get a cheap skate helmet for under $50 and use that. I would sell a full-cut ballistic helmet and put the money toward what I want. Chopping used to make sense when helmets were more expensive, I've done a fair few of them as well as CVC shell conversions but they just aren't economical anymore with plenty of $100-300 ballistic options out there.

Well, I would say this about that.  First of all, if the prime directive is just a NV mount, get a bump; it's what I wear 95% of the time.

If you need ballistic pro (for a class or something) there are many more cost-effective options.

If you just want to have fun doing some kind of mod, go for it.  I have done several helmets myself, when ACH's were under a hundred bucks and cutting them down made sense.  But at the end of the day, they're still pretty heavy old tech.  But you can now mount rails n muffs an lotsa other stuff.  

Best way I have found, rough cut with a thin mitre saw, then grind down closer with belt sander.  Final finish with sanding drum on a Dremel.  Re-seal edges with a good 2-part epoxy.  Re-coat with your favorite finish.

These were great mods back in the day.  But they've been over-taken by new tech.  The juice might not be worth the squeeze any more.

Marine apoxy will seal up the cut.

Yep. I've modded an ACH and used marine epoxy to seal the cuts. Worked like a charm.

I've only got $100 in this helmet, so I'm not going to cry if it gets ruined. What's a good place to look for these $1-300 ballistic options? I don't need ballistic rating, I'm more concerned about budget.

+1

If you know how to use tools and you're a handy type of person, it's pretty easy.

Remove edging and mark it.

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Cut it.

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Clean up the edges, seal it, and reattach the edging.

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Done.

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It might take 30 minutes if you take your time.