Solvent traps are designed to catch cleaning fluids and patches that come out the business end of your barrel…if you’re one of those weird people who actually cleans his or her firearms. While that rarely describes me, I got my hands on two of JK Armament’s JK 155 MST kits from Silencer Shop and began the process of legally remanufacturing them into firearm silencers.

Seen above is both a 155R, rifle kit, and a 155P, pistol kit. Plus drill jig kits in 9mm and .30 cal.

Rifle cups have the straight, 60-degree walls (seen at far right in the photo above) while pistol cups have the radial, curved walls (seen at center in the photo above). Should you also remanufacture your JK MST into a Form 1 suppressor, these different baffle cup designs are supposed to work better for the gas velocities of either a pistol or a rifle cartridge. As JK sells cups individually, you can also choose to mix-and-match as you please.

In order to manufacture your own NFA item, you must first file an ATF Form 1, pay your $200 tax, and receive approval. It’s a relatively straightforward process, and these days if you use the ATF eForms system it’s taking only about three to six weeks from start to approval. Compared to the nine-or-so months a Form 4 takes (that’s the application used when you transfer an existing NFA item, such as when purchasing one from a dealer), the time savings alone is a huge reason many people decide to build vs. buy.

Since you will be the manufacturer of this new NFA item, you must engrave the required information on it. That includes your manufacturer name that you put on the Form 1, which could be your actual name or the name of your corporation or trust, your relevant city and state (two-letter abbreviation accepted), a model designation, a caliber designation, and a serial number.

With the paperwork out of the way and an approved registration and tax stamp in-hand, it’s time to start drilling. JK’s drill jig kits include the necessary bushings, bodies, and drill bits.

For my pistol suppressor I chose to drill the bore for 9mm, which meant a 25/64-inch drill bit, and for my rifle suppressor I chose to go .30 caliber, for which JK includes a 3/8-inch bit. If I only got a single JK 155 MST I’d probably just bore it for 9mm and call it good, as the extra 0.0156″ of bore diameter isn’t going to make or break the MST’s performance on 300 Blackout, but it’s important for safe use of 9mm.

While one could obviously chuck up the JK jig in a drill press — or just clamp the cups themselves directly in place and use a drill press without a jig at all — I chose to stick with the simple DIY theme of this kit, so I clamped the jig into a bench vise and drilled the cups by hand with a battery-powered drill.

The hardened steel drill jig bushing fit precisely around the drill bit and kept everything nicely aligned. The bit never tried to bite into the bushing, but made easy enough headway on the hard anodized 7075 aluminum cups, which soon became suppressor baffles.

I sprayed some gun lube in while drilling, which helped to keep things cool and progressing smoothly.

About a half-hour later and I had a complete 9mm suppressor drilled and ready to use.

Without cleaning up the holes at all post drilling — I’ll leave that to firing some 300 BLK through my “JKMST” suppressor — I was pleasantly surprised as how crisp, clean, and straight they all were.

It all lines up!

Without a doubt the most difficult part was drilling through the front cap.

That’s a thick slab of 7075 aluminum in there and it took pressure and lubrication and about five times more time and effort than drilling the bore through the cups.

Me, an intellectual, I slid a trash can underneath my bench vise and managed to catch 95% of the shavings. Almost no cleanup is a huge success.

What should I do first? Torture the JKMST on this ridiculously short Black Collar Arms Pork Sword Pistol Space Blaster? Perhaps a 4-inch .308 is asking a bit much of this all-7075 aluminum solvent trap-turned suppressor.

This 12-inch 6.5 Creedmoor Pork Sword Pistol is more realistic (and sends a 147 grain pill 1,150 yards before it goes subsonic, plus shoots 1/4- to 1/2-MOA, 5-round groups), but is still harder use than I would suggest for any muzzle device made of aluminum.

This! This is more like it. A 9mm carbine is the perfect platform for a JK 155 MST turned into a silencer, whether it’s a 16-inch barrel as seen above or it’s a large format pistol with a stubby little barrel. This is a great choice for an aluminum, Form 1 suppressor.

With eight baffles, my JKMST is easily configured for different firearm and ammo combinations. Seen above with a Gemtech booster installed thanks to one of JK Armament’s thread adapters, I’ll absolutely be testing this bad boy on a 9mm handgun.

JK’s mount tube is threaded with the increasingly-common 1.375×24 thread size, which means there are many dozens of compatible muzzle devices, boosters, and adapters available from many different companies. No matter what firearm you might be using a Form 1’d JK MST on, you’ll have zero issues finding multiple mounting solutions available for it.

Should you already own a pistol booster or other silencer mount that you like and want to use on the JK, it’s nearly certain to work with the correct adapter (or no adapter, in many cases).

Turning a solvent trap into a firearm silencer process recap:

• File an ATF Form 1 and pay $200 NFA tax
• Receive Form 1 approval
• Engrave relevant manufacturer information onto the most permanent, least likely to get damaged, necessary-for-use-of-the-suppressor part of the to-be suppressor. You can also do this prior to the Form 1 process if you so choose.
• Drill the bore hole through each of the cups and the front cap, which JK Armament makes amazingly simple with their drill jig kits and components.

That’s it for now. Soon I’ll be hitting the range with both my JK 155P MST and JK 155R MST, both of which have since been turned into JKMST silencers, and will follow up with a review. Expect testing on various 9mms, 300 Blackouts, .22 LRs, and perhaps more.

Find more info about the JK 155 MST kits at Silencer Shop and on JK Armament’s website.


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