We’ve all see the guy at the range with his newly, self built, AR15 that looks like Dr Frankenstein had his hands on it. One that seems to have the parts meant for about 7 different purposes crammed onto one rifle. You know, that one we all secretly laugh at behind the guy’s back. I recently had one of those in our shop, here at Crusader HQ. I’m going to go through some do’s and don’ts if you have the drive to build your own AR. There’s noting wrong with wanting to build one. They are fun to build and then you get that sense of pride knowing YOU did that (unlike what our Dork in Chief says).


For some reason everyone wants to have mixed brands of receivers. I know this one waiting to be picked up does. This is a big mistake. Let me explain. We’re playing the game or “tolerance stacking”. This is what you get by going with different manufactures. Every AR15 receiver made is going off of the original blueprints for everything but ascetics, which is great. The “tolerance” is, a specific dimension should be “X” length + or – .0005″ or so. That + or – is the tolerance you have to play with. The tighter to length “X” the tighter your tolerances. Some manufactures get as tight as .0001″ and that is preferred. The design was meant to work properly with everything at the correct dimension but the tolerance is where you can fudge the mark a little. Some manufactures do everything at the high end, some on the low. So if X= 1.025″ but the tolerance means they can go .0005″ over that they will put everything .0005″ over. Some will go .0005″ under so if you put those parts together you have a “tolerance stack” of .0010 off of the original mark. That doesn’t seem like much but over the course of every part in the gun you can be off by almost an inch and that rifle won’t function worth crap.

So with the Frankenbeast in our shop now we have a Brand X lower and a Brand Y upper. Which end of the tolerances are they and how much of a stack to we have just in the receivers alone? I couldn’t tell you. How much is this going to effect the function of the rifle? Depends on how large the tolerance stack is. This is why it is important to go with a single brand of receivers. If they sell them in a matched set GO FOR THE MATCHED SET. They will cost a little more because someone has to match up receivers by hand until they get a nice tight fit. It’s worth the money….it’s worth the money.

We have a little leverage when it comes to internal parts. Get a parts set. All one brand…all the same tolerance so our stack is at zero. If you want a different trigger (Timney, JP, or others) you’re just fine. You saved enough with the parts kit to allow for the probable .0001″ stack you just created. 1- 10,000th of an inch in the trigger won’t give you a noticeable difference in function. It would take laser measurements to tell a difference in function so don’t worry.

Bolt Carrier Group

This is a simple one. Get a BCG from a reputable company. Young’s MFG, Stag, Noveske all good quality. In fact if you have one made by a brand you recognize (preferably the same as the company that made your receivers, remember those tolerances) you should be just fine.


Stocks have become very purpose driven. You have your carbine stocks of every brand and make. Aside from some small differences they’re all alike, as long as you get one that matches the size of your buffer tube (or receiver extension as they are technically called). Grab a carbine stock you like and go for it.

Then you have target/sniper stocks like the Magpul PRS. I really like this stock and it is standard on both our long range rifles. Adjusting for length of pull and cheek height makes this stock a very personal piece of equipment. You adjust it where you are most comfortable and leave it there. Now it is always where you want it to be and where it will help you with that cold shot when it counts.

Another very purpose driven stock is the Magpul UBR. (leave it to Magpul to come up with another stock with a niche) This stock was meant to be a sunk in tight tactical stock. Try to play around with it like you would any other adjustable carbine stock and it will pinch and if you’re the Ogre…you’ll bleed. Think of how tight and compact you need to be if you’re kicking in doors or in SWAT type tactical situations. Elbows have to be tucked in so they don’t snag on something. You are “Vulturing” (Massad Ayoob calls it) into the stock. Once you are in the proper stance the UBR is brilliant.

Now, you mix up a tactical…quadrail with a sniper stock and target crowned barrel…that just looks dorky. Ok, I know there’s a few of the big boys doing it and it looks dorky on their rifles too. You just have two parts for two distinct purposes. Match up the sniper stock with something that you can attach a bipod to…win. Yes you can do that to a quadrail and that’s why the big boys do it. That and they don’t have to by more than one type of handguard. There are other options out there.

Quadrail with a carbine or UBR stock…money. You won the prize. Now you have several options out there for these rifles as well. Even something that would look better on a sniper configuration will work well on a carbine because “you don’t need all those lights and lasers” you can do a better job than guys with all those crutches. You’re still winning.

Anyway, for those of you that want to take the plunge…now you can do so without having a Frankenbeast that your friends will make fun of.

Carry on.

One thought on “FRANKENBEAST

  1. I thank you sirs for this most excellent information. Thank you Sir Joseph of the Chetwood and Lord George of the Hill.

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