FN Five-seveN vs RTAC RT15 vs Kel-Tec R50: The 5.7x28 Question

Three ways into a cartridge that is light, flat-shooting and mild in recoil, and the honest reason most people should hesitate before buying into it.

Why 5.7x28mm keeps coming up

The 5.7x28mm cartridge was developed by FN and is best known from the P90 and the Five-seveN pistol. It uses a small, light, high-velocity bullet in a bottlenecked case, and among its appeals are mild recoil and high magazine capacity. Those two properties explain the rest of this page: they are why people want a gun in this chambering, and why the 50-round carbines in this comparison exist.

The three guns here fill different slots. The FN Five-seveN MRD is a duty-style pistol. The RTAC RT15 is an AR-style rifle listed with a 50+1 capacity and a 16-inch barrel. The Kel-Tec R50 is another 50+1 carbine, with a barrel of about 16.1 inches. We have not tested any of them, so this is a desk comparison built on listings and general knowledge.

The caveat that comes first

Price the ammunition before you price the gun. 5.7x28mm is a niche cartridge. Compared with 9mm, ammunition choices are narrower and the cost per round is typically higher, which makes the gun more expensive to shoot a lot. If your goal is cheap volume practice, 9mm will deliver more rounds for the same money. If you are curious about the cartridge, own one of these and accept the cost, 5.7x28mm is a fun, low-recoil way to shoot. Go in with eyes open.

The three, side by side

FN Five-seveN MRD: the reference pistol

The FN Five-seveN MRD is the pistol that defines the cartridge in the civilian market. The listing shows a 20+1 capacity and a barrel of about 4.8 inches. The "MRD" designation is FN's name for the optic-capable version, so confirm what optic mounting the specific listing provides. For a deeper look at the pistol itself and its place as a carry option, DefensiveHandgun's 5.7x28 for concealed carry is a candid assessment, and their Kel-Tec PR57 versus SIG P365 piece places smaller 5.7 pistols alongside a popular 9mm.

The honest case for a Five-seveN is that it is a light-recoiling, high-capacity pistol that some people simply like to shoot. The honest case against it is the ammunition cost and that a 9mm pistol of similar size offers nearly the same shooting experience with cheaper ammunition and far more choices.

Pick it if you specifically want a 5.7x28mm pistol and accept the running cost. Skip it if you want a general-purpose first handgun.

RTAC RT15: the AR in a different cartridge

The RTAC RT15 is a rifle with AR-style ergonomics built for 5.7x28mm, listed with a 50+1 capacity. The appeal is familiarity: if you know the AR layout, the controls feel known. The question to ask is how much of the AR ecosystem actually carries over. A rifle chambered in 5.7x28mm cannot share magazines or many internal parts with a 5.56 AR, so verify which accessories, optics mounts and furniture fit, and which do not.

A 50-round capacity gives you long strings of shooting with minimal reloads, but ammunition cost is again the limiting factor: emptying a 50-round magazine is a fine afternoon activity until you total the ammunition bill. Check the manufacturer's page for details on controls, handguard and weight.

Pick it if you like the AR layout and want 5.7x28mm in a rifle. Skip it if your priority is a lightweight, packable carbine.

Kel-Tec R50: the quirky carbine

The Kel-Tec R50 is Kel-Tec's 50+1 carbine in 5.7x28mm, with a barrel of about 16.1 inches and a green finish in this listing. Kel-Tec builds distinctive guns, and the R50 follows that tradition. Our Kel-Tec lineup guide explains how the brand's designs tend to trade conventional layouts for compactness and capacity. Check manufacturer details on weight, folding or storage features, and the magazines.

Be aware that Kel-Tec's niche designs can have narrower aftermarket and parts support than a mainstream rifle. Before you buy, check what spare magazines and accessories exist, and read current owner and manufacturer information on reliability, since we cannot make claims about it.

Pick it if you want a compact, high-capacity 5.7 carbine. Skip it if you want deep aftermarket support.

What the PCC comparison tells you

If you are looking at these guns because you like pistol-caliber carbines, our PCC buyer's guide explains the category: easy shooting, cheap practice and shared magazines. The 5.7x28mm trades away two of those three. You keep the easy shooting, and you give up cheap practice and shared magazines. That is not a verdict, just an accounting.

A worksheet for the ammunition question

Before choosing a gun, run this small exercise. It takes five minutes and prevents most regret in this category.

1. Decide how many rounds you realistically shoot in a month. Be honest. Most owners shoot less than they imagine when they buy.
2. Look up the current per-box price of 9mm practice ammunition and of 5.7x28mm ammunition at the retailer you would use, and divide each by the rounds per box.
3. Multiply each per-round figure by your monthly rounds, then by twelve.
4. Compare the two annual totals. The difference is what the cartridge costs you for the privilege of shooting it.

If the gap makes you wince, the gun will probably sit in the safe, and a 9mm pistol or carbine would serve you better. If the gap is something you are comfortable with, nothing about the cartridge is a mistake. The point is to make the decision knowingly rather than discover it after the first couple of range trips. We do not print prices here because they move constantly, but you can do this arithmetic in a few minutes at any retailer, including through the category links on this site.

Optics, sights and setup

All three guns invite an optic, and the setup question differs by gun. The Five-seveN MRD is the pistol to check for slide optic compatibility, since pistol optics use different mounting footprints; our article on red dot footprints explains why matching the footprint to the optic matters. Rifles need a mounting method that suits their rails or receivers, and a rifle in an unusual cartridge may have a narrower choice of compatible mounts than a standard AR. Whichever you choose, zero at a distance that suits how you will use the gun, and verify the zero after any disassembly.

One more practical point is the trajectory of this cartridge. 5.7x28mm is a flat-shooting cartridge at modest distances, which makes zeroing at a close distance workable, but the exact numbers depend on the load and barrel length. Use the ballistic information published by the ammunition maker for the load you buy, rather than a rule of thumb.

Decision guide

Before you buy

Confirm the legality of the gun and its features in your state, including magazine capacity. Check that ammunition is available in your area and compare the cost to what you would pay for 9mm. Plan secure storage. And consider renting a Five-seveN or similar 5.7 pistol if a local range has one, because ten minutes with it will tell you whether the cartridge's appeal is real for you. For help deciding where a niche cartridge fits in a collection, our collection-order guide sets out a sensible sequence.

Editorial note: This is an independent editorial overview built from manufacturer specifications and publicly available information. We have not performed hands-on testing of the products discussed, and nothing here is a performance guarantee. Verify specifications, compatibility and legality for your own situation.

Affiliate disclosure: Some links on this page are affiliate links to Ammunition Depot. We may earn a commission if you buy through them, at no extra cost to you. Our editorial choices are not influenced by commissions. Check current price, stock and specifications directly with the retailer.