My experience making an 80% SIG P320


A while back I picked up one of those surplus M17 pistols. I kinda liked it. I feel it is slightly inferior to the Glock, but not terribly so. I wanted to try the compact M18 pistol, and I ended up figuring the most expensive way to do so.

Making your own “Ghost gun” from an 80% complete receiver is growing in popularity. What could be more American than making your own gun? It is kinda fun to do, but I feel many of the arguments for doing it are foolish.

Cost? It will probably run you somewhere from 30% to 100% more to make a gun your self from an 80% kit than to buy a complete one. This cost will vary per gun, but certainly it is not cheap to make your own P320.

Untrackable? If you buy a gun with cash from a face to face sale there is no paper trail. If you buy an 80% kit online the company who sold it to you has your info, your credit card company has recorded a transaction. UPS/Fedex/etc would have a shipment recorded in the system. It would be very easy for the goverment to ask Visa and Mastercard for the info of everyone who purchased from a company making 80% receivers. They could pull the shipping logs, etc. See who has been purchasing parts kits online. Hell, your 80% gun might have more of a paper trail than had you just bought it from the dealer.

Quality? Face it, most of us don’t have machine shops laying around, and all sorts of precision equipment. It is easy to screw up an 80% frame and just be out the cost. You have no warranty or support if you go this route.

Why make a gun from an 80% receiver? Because we can.

I picked up an 80% P320 “MUP-1” receiver made by JSD Supply for $100. The jig to complete it cost $150. That is half the cost of a new gun right there. As parts are in limited supply, I had to buy a parts kit which also cost me about the cost of a new P320 pistol. Not a financially smart way to get a hand gun.

The 80% frame is stainless. You have to bend and trim the frame rails that the slide runs on. Then you need to drill all the holes, relieve some material, and cut a slot.

You use their jig to line up the holes, and to bend the rails. It includes a cheap set of drill bits. I broke two of the drill bits while attempting to drill the stainless steel receiver. I found the cheap drill bits I keep at home cut so very much better than what was included with the kit.

I’m not going to go into all the details, or show instructions on how to make the receiver as all that info is already available with the kit.

I choose to make the kit using a vise, files, electric drill, and a Dremel tool. It would have been much better to use a drill press instead of a hand drill, but I wanted to see how it would turn out with a hand drill (spoiler, poorly, but works). I feel that if I had used a Bridgeport mill, I could have had it done in 30 minutes (aside from deburring) and made it perfectly.

It took me 4 evenings to finish the receiver. The first evening I bent the rails and drilled a couple of the holes. The second evening I drilled all but one hole (because I broke that size drill bit), did the inital trimming of the rails. Deburred the holes. Third evening I thought I had it done, but when I went to assemble it I had a fair bit of fitting issues, so getting everything fitted together right ended up making it take another day.

I have a little arbor press. I thought that would be perfect for bending the rails. Didn’t work well. I ended up putting the jig in my Wilton bench vise and that made bending the rails quick and easy.

Looks terrible, but it works!

Despite the jig, using a hand drill left me with several crooked holes. I ended up drilling out many of these holes much larger to give enough slop for the cross pins to fit. Funny enough, the P320 design easily accepts and works with horribly enlarged pin holes. I imagine this would not have been necessary had I used a drill press.

There is a slot you have to cut between two holes on the left side of the frame. I used a Dremel. The first Dremel cut the slot half way deep easily then stopped cutting. I think it work hardened the stainless and then got dull. I struggled a bit with a couple of ceramic bits and got the slot cut.

Once again, let me state how amazed I am at how much the P320 design can deal with slop and misshapen holes and slots in the frame.

The M18 parts kit I bought is set up for a thumb safety safety. The MUP-1 80% is not cut for a thumb safety. This lead to a good bit of confusion on my part when I was assembling it. I have it put together with the safety, but the safety isn’t properly retained. I’ll need to pick up some replacement pins and a grip to convert it to a non-thumb safety setup.

I looked around for the replacement parts for setting up the gun with out a thumb safety, and they are out of stock everywhere I looked. If you decided to make a P320 from a 80% frame, make sure the parts kit you buy is not a thumb safety model.

The instructions I was using for the 80% frame told me to cut the frame rails to the same width as a factory P320 frame. It did not give a size. I wanted to see if I could get it set up with out measuring the factory gun as that may not be an option for some. Doesn’t do you much good if the instruction for your gun say to get another gun of the same type for reference.

That caused me a problem. On the 3rd evening I was working it, I had filed down the frame rails until they fit into the slide. I smoothed out the rails with some Nicholson files (now made in Mexico and China, no where near as good as the old Nicholson files). My made in Mexico Nicholson file did easily cut and shape the stainless steel of the frame and make working on the frame rails easy. I left the frame rails as wide as I could for as tight a fit as possible in the slide. That was my problem. While it all assembled fine, the rear rails were slightly pinched together by the slide once the other small components of the frame were added. This pinched the mechanism and caused the gun to lock up. I had to use a rubber mallet to hammer the gun apart. Narrowing those rear frame rails solved the problem quickly.

I don’t want to say making the 80% SIG P320 was hard. But it wasn’t easy either. Not like the 80% Glocks.

The P80 80% Glock is very quick and easy to make. But if you screw up the 6 holes drilled for the 3 pins, you might as well scrap that frame and start over. The 80% P320 on the other hand has so very much more work involved, but if you mess up, it is far more forgiving.

I hate to write it and admit it, but my half-assed sloppy and shoddy work on the P320 receiver was so bad I didn’t expect it to work.

That is why I was caught off guard when I found my home made receiver has a nicer trigger pull than the factory M17 trigger. I have the military surplus M17 and my home built M18 pistols side by side and the trigger was so much better on mine. (Mind you, the thumb safety was sloppy and while it functioned, it didn’t click into place because the aforementioned issues).

I shot the two guns side by side. I swapped slides, frames, and grips between the two to check function and it just worked.

A few thoughts:

The rear sights on the milsurp M17 and the commercial M18 kit are rather different. I much prefer the rear sight on the milsurp M17.

I find I prefer the handling of the shorter M18 slide, but I shot so much better with the M17 slide. I don’t know if this is just due to the longer sight radius, of if there is some other reason.

The full size M17 slide often does not want to completely return to battery on its own. I find that terribly unacceptable. The M18 does not have this issue. The recoil spring assemblies are very different between the full size and the compact.

There are several differences between some of the small parts on the military P320 and the commercial one. Mostly trivial, but the slide stop is much smaller on the commercial model and much harder to use. I am going to have to find a replacement one that is the same size as the one on the M17.

I’m going to write more about the M17 vs M18 later, and go into detail then. I more just wanted to share my experience with making a pistol from a 80% frame.

I think the smart thing to do is just buy a factory made pistol if you want one. But if you really feel like making an 80% gun, go for it. Just don’t delude your self into thinking it is untraceable or a good dollar value.

Sometime I’d like to make another one of these, where I’d use a milling machine and make it perfect. Set it up to use the thumb safety. But, until then, this one works fine, and all the egged oversized holes and file marks are on the inside of the gun where you can’t see them. And it still has a nicer trigger pull than my factory Sig pistol.


    • Not hard to drill straight holes, and if you can’t do it in a piece of wood first just stick to buying assembled guns from the manufacturer plain and simple. Drill speed is key for stainless and not going into all that as what material your drilling dictates the speed you need the bit to run at and how fast you feed it also.

  1. Nice,like these builds as stated if all cash really tis a ghost but also gets folks more into the whole process of how their firearm works.

    I know a few folks who did the poly 80 glocks that are happy with them.They bought em with blank grips and stippled them to their desires.They bought em pre corona beer panic @ about 120 with a lower parts kit,then,just a loaded slide or parts,ran about even to a hair more then a new complete buy.Now the kits and parts/slides ect. just really over the moon,but then,what is not in the ammo/firearms world.

  2. Thanks for this write up, Howard. So I can grade your assessment on a curve, please remind me: you are some kind of machinist, right?

    • Hey John.
      I’m not getting any notification for comments. I gotta figure out what is wrong and fix that.
      Yes, I am a trained and certified Machinist, what what little that is worth.
      I intentionally made this P320 frame with just a bench vise, power drill, Dremel, and files to see if it could be done, and I found it does work. But it should be done with a drill press. I thought the sloppy hand drilled holes would not work, but it does. The P320 design is very forgiving of slop apparently.
      I thought the P80 80% Glock was extremely easy to do. This was considerably more work, but certainly not out realm of what someone could do in their living room over four evenings like I did.

      • Howard . Do you have a blue print you can share , or want to sell your jig? As you I am also a machinist by trade. I cant find a jig any where . Ive also noticed that the firearms seem to come out better when a print is followed. My email is thanks Mark

  3. I have completed 2 Sig builds a X Compact and X Carry/Sub Compact getting ready to do another X Compact I did use a drill press añd purchased some COBALT bit’s because drilling in the MUP 1 is no walk in the park the 316 stainless steel is very hard steel and when I say hard it’s hard, my experience with these build’s is with each one you will get better not to brag but my 2 builds are just as good if not better than the factory p320, it is kinda expensive but doing quite a bit of research will save a few bucks, yes one can no doubt buy a factory gun cheaper than building one (Sig) for me it’s satisfactory knowing that I put every single part together to make my own gun it produces a awesome feeling and lots of pride go along with it, I just love doing these Sig builds.

  4. Glad to see the review. We do make manual safety cut mup-1’s that come pre cut for the manual safety controls. I don’t remember the exact date we started offering them but it must have been after your purchase.

    I would like to point out acouple things that will maybe help some others.

    1. you should always use a drill press. It prevents damage to the jig and gives straight holes.

    2. For the rails, if you cut them with a hacksaw right against the top plate they will be almost the perfect width. I’ve taken some receivers out of the jig and slapped a slide on and they worked right away. Most of the time you’ll want to leave them a hair wide to fit them to your slide.

    Overall I’m glad to see the review!


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