SIG P320 Series Part 4 Build tips, recommendations & jig. Final Prep for the Sig videos

Updated: Jul 6

Contribution from Capt'n Crabby Well, couldn't wait for all the videos from MGB ... Patience is NOT on of my virtues 🤣


Overall it went well - between MGB's rail bending video and the drill guide, I got 'er done. Super happy with it, but I think I'm going to do just a tad more sanding on the rails, it is just a tiny bit tighter than I'd like, a tad bit more than a break in might fix.


I'm not a wordsmith, I don't have deep wisdom nor a good way of explaining things - thats why I was a knuckle dragging worker grunt. But if I was going to make any recommendations, it would be:

  • Watch all the videos you can -MGB and others like Rogue Banshee (or something like that) did a series on the 320 80%

  • Read and comprehend the Drill Guide on MGB's blog. Then read it again.

  • Get the carbide drill bits - i cannot even tell mine have been used, and now I have extra sets that will just sit unused. 😉

  • LOTS of Tap Magic cutting fluid

  • Go sloooooooow and make damn sure your drill bits are perfect in the jig holes

  • Spend LOTS of time with a file removing any burrs or jaggies from the drill holes

  • Sand EVERYTHING - started with 800 grit, moved to 1500 and ended with 2000. I sat in front of the TV binging The Clone Wars and every other episode I changed grit. It is now shiny smooooooooth!

  • Rails rails rails. Spend time on them. File them slowly, test fitting often. Be sure they are even on both sides.. And then sand sand sand. I have 2 slides. Each will have a FCU specific, so right now the FCU that I'm about done with will NOT work in the other slide. That way the fit is better, otherwise I'd have to have one FCU fit both and one would be better fit than the other.


Cutting the Rail Ears P320



Be careful and go slow opening up the area between the 2 small holes, where you remove the metal and make an oblong opening. Don't remove any material other than between the holes - don't let the dremel wander up or down, just go between the holes. You want the hole oblong with nice straight edges, not more circle like. I did it freehand, and it bounced on me a few times. Amazing how much it can remove in a blink of an eye, when you don't want it to 🤣 Otherwise its a slow tedious removal. It ended up good, but there's a spring behind that opening and if you open it too much, I'd hate to have that become an issue.

I used the dremel workstation for opening the space between the 2 #40 holes. There is no way you do this project without a drill press.

MUP-1 rail bending

Contribution from Capt'n Crabby , MGB & TonyHunter B


So, this always happens to me. Is it me, or is the rail on the right not bent enough? It won't go anymore, I've put a few extra whacks on that corner and it won't move any more. The other three are equal to the left side rail in the photo.


Flip the jig top back to vertical facing inward and fix the angle. Then go flat again.


Ok, that seemed to have done it. Follow up question... I assume the tabs should be perfectly flat/horizontal, more so than the left side in the picture? Or is the left tab in photo gtg? All 4 equal the left in the photo. Sorry for the 20 questions... I'm the kind of person that cannot draw a straight line with a straight edge, a plan and a month to work out the details. I've beaten on it more than the 10-12 whacks per end like the video... LOTS more.


Should have only been 6 firm hits. The secret is when you are vertical to make sure those ears are well passed the top plate before seating it horizontal.


I have used a 3/16 flat punch to hammer down the rails completely flat. The metal that the punch contacts gets cut off anyway, so no harm.

Thats exactly what it did... worked like a charm. Once I realized they were not going to go flat in tge jig without a LOT of pounding, and the ends git cut off it seemed tge punch was the ticket. Thanks for reinforcing that


You can see in the first pic where I hit the rail ends with the punch. 2nd pic shows the results. FLAT!

Another member ahead of the videos. Getting Started !

Contribution from Tom Robinette

Holes are drilled and tabs are bent. I believe it’s time to start cutting.

So far I have learned you need a decent size hammer not the little one in your kitchen drawer and slow and steady and cutting oil when drilling. The worst thing you can do is get in a rush. You need to drill a lot of holes and it’s going to take a little while and that’s ok.


Something that wasn’t really clear to me at first was how you end up drilling into the insert for some of the holes. This makes setting your depth on each hole very important because it’s not always easy to tell when you are through the FCU and into the insert, so setting the depth of plunge is the only way you know to stop.


I still have a lot to learn as I move forward but I’m happy to answer any questions I can.

Getting the rails to the exact measurements.

Contribution from ashere814


This is one of the most important parts of the Sig build getting the rails to the exact measurements of 0.857in. for the front and 0.859in. for the rear I use the small detail files for this going slow and checking the measurements often (also the corners of the rails need to be rounded off)a caliper is almost a must for this to get the exact measurements and remember after the measurements are achieved the rails need to be polished to a luster shine when I get through doing a Sig build i expect it to be better than one purchased from a gun store in other words I want it to be First Time Quality 🇺🇸🔫🇺🇸


This is measurements from a factory FCU














Note: MGB's plan to make this process easy peasy for you. The jig as presented on 3/3/21 Wacky Wendsday video below.

Contribution from ThePyrodex & photo by Cory Sapada

Its a jig to help make cutting the tabs a breeze. A colaboration between MGB, ThePyrodex, John Cromwell, & Tilly.


The basics you pop the new jig into the MUP, it only fits in one direction.


Now make sure its flush and flat to the tabs and take a sharpie and trace the edges of the jig onto the tabs, pop the jig out and rough cut them up to the line. I personally just used some hand files but you could also rough cut them with a hack saw ( I avoid using power tools when possible, the work fast but also mess things up fast lol)


Now pop the jig back in and file right up to the edge of the jig doing your best to keep the file 90 degrees to the jig (should be pretty easy as you can use the jig to help with this part)


Now you take your slide and check the fit and see if it goes in easy, Mine needed a small amount of hand fitting as there was 2-3 areas that needed a pass or 2 with a jewelers file to take off a bur or flatten a small high spot.