Rock River Arms LAR-15M .17 HMR: Full Review

By Tom Beckstrand

The .17 HMR exists because an old man got tired of blasting pests off his birdfeeder with a .22-250 Remington. The elderly gentleman in question had a son who worked as a ballistician at Hornady. One day, he got to complaining about the recoil and muzzle blast his .22-250 made and asked if there was something better. His son, Dave Emary, started tinkering with necking down .22 Winchester Magnum Rimfire (WMR) cases to .17-caliber, and the .17 Hornady Magnum Rimfire (HMR) was born.

Contrary to what many believe, the .17 HMR isn’t just another rimfire cartridge that can be made to work in any blowback action. The .17 HMR was designed around a powder called “Lil’Gun” because it has the best burn rate for lightweight .17-caliber bullets. Using any other powder produces poor velocity. The problem is Lil’Gun powder doesn’t play well with blowback actions. Given their desire to build a reliable .17 HMR AR-pattern rifle, Rock River Arms (RRA) set off on a two-year journey before finally creating the LAR-15M .17 HMR seen here. It is a brilliant solution on how to make a semiautomatic rifle function flawlessly with the .17 HMR.

(Photo by Mark Fingar)

Gas and Ball Bearings

The reason it took Rock River two years to develop this rifle is they had to create a gas system and a bolt carrier unlike anything in the firearms industry. The gas system is direct-impingement and about two inches long. Once the bullet leaves the chamber, it travels down the bore and past the gas port, just like what happens on the big-boy AR-15. Gas moves down the short gas tube and into the receiver where it enters the gas key atop the bolt.

gaad-rra-17hmr-1200x800-03 — For this .17 HMR AR, Rock River built a gas system and bolt carrier unlike anything else on the market. (Photo by Mark Fingar)

The bolt in the LAR-15M .17 HMR looks like a miniaturized bolt carrier group. The bolt is one piece of 8620 steel that is plated in electroless nickel and is a little over six inches long and about a half-inch in diameter. When the gas hits the gas key atop the bolt, this is where the magic happens. The gas key moves rearward, independent of the bolt. Once it moves about ¼-inch rearward, two large steel bearings that protrude from the bolt body into the receiver walls retract, and the bolt moves rearward together. The movement of the gas key delays rearward movement of the bolt enough for chamber pressure to drop while still giving the bolt a hard-enough shove to cycle.

The bolt and gas key assembly weigh in at seven ounces, so the gas system has to give it a solid push to get it moving. Unlike an AR-15, there is no buffer weight in the RRA .17 HMR, just a thick plastic spacer and a buffer spring. While the gas system, bolt, and operation bear a passing resemblance to a traditional AR-15, this .17 HMR rifle continues to blend rimfire and AR-15 attributes seamlessly together. The extraction and ejection of this rifle are classic rimfire with dual extractors sitting at 3- and 9-o’clock on the bolt face. Timing cuts correspond in the breech face of the barrel. There is a fixed ejector bolted to the inside of the upper receiver that kicks empty cases out of the rifle with ease.

gaad-rra-17hmr-1200x800-04 — Clear 10-round magazines are oversized and made by Black Dog Machine. (Photo by Mark Fingar)

The ball bearings that lock the bolt in the closed position are an unusual feature. This delayed gas impingement system with the use of ball bearings is a patented and licensed design from the original designer to RRA. Although this design was thought to be ready for immediate release, the folks at RRA had to do a substantial redesign of all major components from different materials and even revise the heat treatment specifications. It would be rare that any design ever shows up with full print package with everything correct, and this one was no different. RRA spent two years in redevelopment and testing prior to releasing it in 2025.

The bearings sit about two inches behind the bolt face and are under opposed spring tension. The spring inside the bolt pushes the bearings into recesses in the upper receiver wall, ensuring the rifle doesn’t fire out of battery and that the bolt stays closed for pressure to drop to safe levels before moving rearward.

gaad-rra-17hmr-1200x800-05 — Magpul furniture comes standard, including this MOE SL-K stock. (Photo by Mark Fingar)

While .17 HMR ammunition operates at 26,000 psi, the challenge with Lil’Gun in a direct-blowback system is it still builds pressure after the bolt moves rearward. This could lead to ruptured cases and general unpleasantness exiting the ejection port as well as failures to cycle. The ball bearings hold the action closed long enough for pressure to drop, but steel bearings pushing into a traditional aluminum receiver are certain to create horrific wear on the upper receiver. RRA found an elegant solution by inserting steel cups into the sides of the receiver to house the bearings when the action is closed. This is as thorough an engineering solution as I’ve found on any rimfire rifle and one that pairs the unique .17 HMR’s pressure and performance characteristics well with the AR-pattern rifle.

Why .17 HMR?

RRA did a lot of work to get the .17 HMR to work in an AR-pattern rifle. The natural question is, “Why?” The .17 HMR pushes 15- to 20-grain bullets at anywhere from 2,250 to 2,550 feet per second (fps). Unlike just about every other roundnose rimfire bullet, these are spire-point projectiles with better external ballistics than their slower and rounder counterparts. The result is an exceptionally fast bullet that holds velocity well and kills pests downrange effectively out to 200 yards.

gaad-rra-17hmr-1200x800-06 — Areas like the shrunken ejection port give away this rifle’s special chambering and design. (Photo by Mark Fingar)

The LAR-15M .17 HMR is meant for cleaning house in prairie dog towns. The ingenious operating system makes for a smooth operating yet extremely durable rifle. The 18-inch chrome-moly barrel has a 1:9-inch twist rate and a ½-28 muzzle. My experience indicates that it takes some rounds before the barrel settles in, which isn’t unusual for production rifles. Accuracy for five-shot groups at 100 yards is available in the attached table, but I found this rifle capable of staying under 1.5 MOA. I had gusting winds at the range, and most of my group’s dispersion was horizontal. Any rimfire at 100 yards will see the effects of wind long before a centerfire will.

gaad-rra-17hmr-1200x800-07 — Takedown protocol for the LAR-15M is just like any other AR. (Photo by Mark Fingar)

This rifle takes 10-round magazines made by Black Dog Machine, and they are great. They’re clear to show round count and big enough to handle and store easily. The rest of the rifle is what I’d expect from any quality AR manufacturer. The handguard is 15 inches long and free-floats the barrel. The lower receiver is the standard forging and comes with RRA’s ultra-match trigger, which is a joy to shoot. The upper receiver is made from billet to allow for the machine work required for the steel inserts that house the ball bearings. The upper receiver group is highly specialized for the .17 HMR, but it is available for purchase separately and will drop onto any standard AR-15 lower. The Black Dog magazines will work in the same standard AR-15 lower receiver.

This rifle is a dream come true for any rimfire enthusiast. It leverages all the things we love about the AR-15 and brings them to the .17 HMR. We get the fast-handling and quick reloads of the AR with the extended range, great accuracy, and superior terminal effects of the .17 HMR all in a reliable and durable semiautomatic rifle.