It's a pretty neat idea for the hobbiest, and may work out to be completely reliable pistol, but the 100 versions of available Glocks are not really expensive as handguns go (and pretty utterly reliable) (and there are always about 100 used of all shapes and calibers for sale on my local gun boards), and if I was trusting my life with one, I rather have one assembled by Glock, than me. A good man knows his limitations.
I had been waiting to post a thread on my new go-to primary home defense pistol (until I had a chance to shoot it), but the modularity issue of this Glock frame is very similar to the new(er) Sig 320, so here goes:
The US Army has adopted the Sig 320 as it's new pistol (although Glock has appealed the decision, they will lose). I have a lot of former US military firearms, but never liked the Beretta 92 (too large a grip, too far to reach the trigger on the first double action shot). The Army put out a contract wish list for a modular pistol that can be changed around to a number of sizes and calibers with ease and small expense, and Sig changed up it's model 250 to the 320 to meet that contract.
Here is the full size gun I purchased. (very similar feel to the old 1911)
See the serial number peeking out from a hole on the ambi slide release? It is stamped on the only part of the firearm that is federally documented as the actual firearm. For ATF purpose, this is the gun (it combines both the trigger group and the striker mechanism into a single part, that can be dropped into any size frame. Virtually every other part of the pistol can be purchased over the counter with no FFL paperwork necessary. Also notice, it is a metal (not plastic) trigger, with no little lever in the middle of it. And this striker trigger is one of the very nicest and sweetest with light pull and really short (and tactile) reset of any in the business. Most of the reviewers place it much better than all stock Glock and Smith triggers, and about on par with the FNs and Walthers.
They are being offered in full size, compact and subcompact (similar to Glock) in 9, 40, 357 Sig, 10 and 45 calibers. There is nearly complete interchangeability of parts, except the 45 was too large to be fully interchangable. You can buy assembled parts kits from Sig to change pistol A to B, either size of gun, or change of caliber, or both.
The one mistake I see is calling the full size grip frame and mag with shorter bbl the
carry model. The butt is always the hardest thing to conceal, so IMHO a carry model should have been reversed with a smaller butt and longer slide.
Also, rather than having 3-4 different inserts for the back of the grip frame like most of the other makers of striker fire duty weapons do, to accommodate different hand sizes, you can just order a different grip frame size in it's entirety (and then just drop the one piece trigger/striker part into it). And those frames are like $37 apiece.
And, if you want an RMR reflex sight on one, you can buy one with Sig's own proprietary reflex sight (Romeo 1) attached (unlike the other makers, where you must buy your own aftermarket reflex and special base to mount it to your particular pistol separately) Sig's reflex gets good reviews. And if you get their pistol with reflex sight, it comes with the taller fore and aft iron sights for co-witness with the reflex (which have to be added to other pistols).
Now with a lifetime of shooting and collecting behind me, I was not interested in buying a base gun, and then buying a bunch of parts to change it into all manner of Leggo-part different guns. I already have my small carry pieces in auto and revolver. But when my long serving primary home defense, full-size, hammer fired, all steel, double stack (but no picatinny rail) 9mm night sights lost their tritium glow, and my local shop said they would not change them out as they are particularly tricky, I decided to change up to a full size striker fired duty weapon with a rail for mounting lights and lasers.
For home defense (and not carry) I looked carefully at the longer bbl full size frame target pistols offered by Glock, Springfield XD, Sig, Walther, FN, and Smith and Wesson. I was thinking about a reflex sight too, but decided I'd rather have a combined light and laser added to the rail underneath, with just night sights on top. I also wanted a very grippy polymer frame (and here, the new Smith 2.0 models have the nicest grippy frame I've ever held in my hand), but the P320 is also good and grippy (without sending it off for expensive hand stippling).
I really wanted a 5" or longer bbl/slide, but the full size P320 is 4.7" and close enough, and the accuracy of this pistol in all reviews on line is very very good. So this Sig won the contest for me. And note, while not exactly cheap, it is hundreds cheaper than their all metal, hammer fired new Legion and Mk25 226 models.
My new 9mm P320 Tactops (with 4 21-rd mags included) (and all full size P320 frames offered) comes with good Sig night sights on the rear, but something I had never seen on the front, called a TFO (tritium/fiber optic) sight. Anyone who has used fiber optic front sights knows that in full or even partial sunlight, the things glow like a green electric neon sign out front, and is super quick to pick up. They may not be better than an old fashioned black partridge front sight for pure target shooting at the range, but for minute of bad guy, they just cannot be beat. Now the really cool thing about them is, as you transition from light to shade to darkness (say stalking or looking for an intruder in and around your property/buildings) the glow of the fiber optic sight just transitions to the night sight as you enter a dark building or basement, seamlessly. And finally, this P320 has the same grip angle and feel as a slightly pudgy 1911 which I love (I do not care for the more angled grips of Glock and others), and without the weight of a 21rd mag, it is only about 2/3 the weight of an all steel 5" 1911.
My new primary home defense.
https://www.sigsauer.com/store/p320-full-tacops.htmlWith this TLR2 G light and green laser added on the dust cover rail.
http://www.streamlight.com/en/products/detail/index/tlr-2-gAs an aside, I do not believe the smallest P320s (12 rds) are necessary the best choice for concealed carry, they would certainly work and it is a fine weapon, but may be larger than many would be happy with for carry. This pistol was designed for military duty in a range of large, medium and small iterations. I won't be packing it anywhere, esp inside the waistband.
Sorry for the thread drift. It is relevant to polymer frame changes without FFL issues, like the Glock above.
Some reviews (look at the accuracy testing table in this one, regular 25ys groups under 2" is damn good).
http://www.range365.com/sig-sauer-p320-gun-reviewhttp://www.thetruthaboutguns.com/2014/05/foghorn/gun-review-sig-sauer-p320/https://www.shootingillustrated.com/articles/2017/1/20/review-sig-sauer-p320-pistol/http://www.military.com/daily-news/2017/01/19/army-picks-sig-sauer-replace-m9-service-pistol.htmlHickok can really shoot, but the pistol certainly helps.
https://www.youtube.com/watch?v=IyBfrf7iitA 
