6x60? is this a powder assisted weapon or is that the projectile size? Gonna be a mother of a recoil if you're firing even a plastic dart that big at hypersonic v.

Actually, it's purely the dimensions of the projectile. I ran this design something like two or three years ago through a system called 3G (Guns Guns Guns). It is almost devestatingly realistic in its' figures for recoil, ranging, effects, power input and output. It required me about 6 hours of math on a scientific calculator based upon the amount of joules of energy delivered onto target at 100m. And yes, the book requires that math or you can't figure anything out on it.
The design itself is meant to be used by either a well-augmented human / furry or a light powered armor, and the rifle weighs in at close to 55 pounds. It's decent, though, in what it was meant for. Oh, as for projectile dimensions? It's based on the amount of intended energy delivered, and has a crudload of equations needed to arrive at it.
If you love math and just want to dull your mind on a hyper-realistic, mathematically-accurate modeling of a gun, 3G is for you.
As a test, BTW, I tried to reproduce the M-16. It came out accurate to within 0.5 oz., and the performance was spot-on with what the reports list it as.
In short, the system is sick-accurate, but then again, for 5 pages of math to define a gun, it bloody well better be.

I've got a couple of questions about this. One, where's the ejection port. Also there's no visible safety either. Are they on the other side of the weapon? Also the stock isn's very well desighned, it doesn't look like it would fit well on some one's shoulder. Also I'm assuming the scopes are monted onto the carrying handle, other wise the gun won't be hitting much past 150mm at all.

You do however have a very good idea when it comes to ammo, it's what actualy makes a weapon, without the proper ammo a gun is nothing. 6mm plastic rounds that deliver kenetic energy and incindiary to a target would be very effective. But as for this 3G thing, I wonder if they factor in the different mechanics of the internal workings of a weapon. It may factor in gas operation, but does it work for delayed blow back or recoil operated. I'd like to know if it would be something I'd consider for a set of highly realistic weapons I intend to make for a game. Great work by the way.

The primary feed is electrically actuated, as opposed to anything fired utilizing an explosive cartridge system. The ejection port IS on the opposite side of the gun from that shown, and is used purely for clearing jammed or damaged ammunition.
Due to the fact of the drawing being a /bit/ too small, I omitted some necessary detail. As well, when this was drawn (approximately 6 - 7 years prior) I hadn't had the experience in actually /firing/ weapons, only in looking at them, thus the design of the stock is, indeed, poor.
The scope, also not shown, is supposed to be extensively computer-enhanced, and indeed would as surmised fit on rails located in the carrying handle area. Again, the design is unrevised, thus I would likely mount it slightly differently currently, likely eliminating the carrying handle and the front raised area (inspired, poorly, by the M-16 again without a full working understanding of the mechanics of said weapon at the time).
On the matter of 3G taking into account different types of action, yes, indeed, it does. For explosive-cartridge ammunition weapons, the feed method and action will affect the overall reliability and effective aiming of the weapon, altering accuracy and range and efficiency. Be advised that just that one section WILL consume some 3/4 of an hour with a scientific calculator and a good page of paper. It is one of the reasons I moved into the directed energy weapons section, and also into the rail/coilgun end: their math was simpler. ^.^
Should you be capable of finding a copy of the book, you will find that it includes passably-effective conversions with a decent degree of accuracy allowing the designs to be translated into multiple major RPG systems common approximately 6 - 10 years ago.
I wish you luck in your efforts.

Yours,
Mika Kyubi
Kitsune-at-Large

Last note: I was incorrect on how recently I had done the math. For this bit of misleading information, I most humbly apologize. Gomen!