Category Archives: AR-15

Federal Adds 90-Grain Bullet to Fusion 6.8 SPC MSR Lineup

Federal Premium Ammunition is expanding the popular Fusion MSR line with a 90-grain option in 6.8 SPC. The load joins the existing Fusion MSR lineup, which includes 223 Rem. 62-grain, 6.8 SPC 115-grain, 308 Win. 150-grain and 338 Federal 185-grain options. Shipments of this new product are now being delivered to dealers.

Modern sporting rifles (MSRs) are the most adaptable class of firearms in history, handling everything from tactical applications to elk hunting. Fusion MSR loads are specifically designed for hunting with these rifles, performing to their ballistic peak through 16-inch barrels for AR15 platforms and 20-inch barrels for AR10 platforms.

The skived bullet tip ensures expansion at long ranges, and a fused jacket around a pressure-formed core produces excellent accuracy. Federal’s part number for the new 6.8 SPC 90-grain Fusion round is F68MSR2 and it lists for $29.95 per box of 20 rounds.

Midsouth Shooters Supply currently offers 112 Federal Ammunition loads. Click here to see when the new 90-grain round is in stock.

Federal Premium Fusion 90-Grain 6.8 SPC
Federal Premium Fusion 90-Grain 6.8 SPC

Reloaders Corner: Pressure Curves and Port Pressure – Part 2

The following is a specially-adapted excerpt from the forthcoming book,” Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order.

Last time I gave a caution about respecting one of the differences between semi-auto and bolt-action rifles, and that was with respect to propellant burn rates. The summary reason for that is that different rate propellants will “peak” at different areas as the expanding gases and the bullet travel through the bore. Slower-burning propellants peak farther, and that means more pressure is available at the gas port location in an AR-15, for instance, as the bullet passes it. If the system is oversupplied, then the system is overworked.

Compared to ideal function when gas supply is delivered as engineered, mistimed peak pressures can result in the bolt unlocking too quickly and excessive bolt carrier velocity rearward. The system just gets hit too hard. The extractor tries to yank the case out of the chamber too soon, before the case is released from its grip on the chamber walls (from being expanded through firing). Spent-case condition shows a measurably more abused hull. Probably the worst popular example of these effects is the M1A. I’m doing an entire column or two on reloading for this beast. Essentially, a spent case from an M1A will show dimensions that don’t seem possible. These come from the bolt unlocking too quickly. AR-15s actually handle excessive pressure better than some other designs.

Always keep in mind that this is all happening in about 2 milliseconds. Average time a bullet spends in the barrel, for most modern centerfire rounds, is 0.002 seconds. Timing is everything.

Keeping in mind the behavior of a pressure curve, which is like a wave cresting, factors that influence the amount of gas-port pressure, using the same load, include barrel length, gas-port size, and gas-port location. When the bullet is sealing the bore, the longer the barrel, the more pressure is contained for a longer time. The smaller or larger the gas port size, the slower or faster the gas enters the system. The farther back or forward the port is located, the sooner or later. Bullet weight is a factor also: heavier bullets accelerate more slowly (and also the reason heavy bullets erode the chamber throat more than lighter bullets).

And, the amount of volume inside the bore has a huge influence on all this. That matters when we’re using another caliber than .224 in an AR-15 or .308 in a big-chassis AR (like an SR-25). For instance, in that rifle chambered for .243 Win., but retaining the gas system specifications (gas port size and location) of the .308 Win.–chambered rifle, there’s way more pressure only because there’s less space, less volume, in the bore. The opposite is usually true when we’re running an AR-15 with a larger caliber bullet.

Selecting a propellant with a suitable burning rate, which, again, is something in the vicinity of H4895, is really the only thing we can do on the loading bench to ensure that we’re not contributing to these symptoms. Beyond that, dealing with excessive pressure gets technical.

All my NRA Match Rifles, which usually have 26-inch barrels, get their gas ports moved forward one to two inches. These, of course, are custom-barreled. I also usually install an adjustable gas manifold.

Moving the port forward effectively delays the wave of gas moving through the bore, kind of repositioning its peak with respect to its outlet; there is more space available for expanding gases. It also allows a little slower-burning propellant, which can take more advantage of the longer barrel. It’s common in a similarly constructed AR-10 to get a port moved as much as 5 inches forward to accommodate a .243 Win. or .260 Rem. chambering.

The adjustable manifold allows some tuning. There are essentially two forms these take. One way is to restrict or limit the through-flow; the other just bleeds it off. I like the first kind the best.

Also, I have searched far and wide for a consensus on gas-port sizes, and came up empty.

All this changes with different chamberings and rifle configurations. Carbine-length barrels are particularly sensitive to port pressure because the port is located farther back.

There are a few surefire things that will alert you when your rifle is exhibiting “over-function” symptoms, such as spent-case condition showing excessively blown (extended) case shoulders, extractor marks on the case rim, and a generally explosive sensation in functioning.

In a more extreme circumstance, an over-accelerated carrier can “bounce” back from its rearmost travel so quickly that a round can’t present itself in time to be picked up by the bolt, or the bolt stop can’t engage quickly enough to hold the bolt carrier.

Sometimes what appears to be a “light” load is actually not. I’ve seen excess pressure leave a spent case in the chamber because the extractor lost its grip, and I’ve seen chunks pulled right off case rims. That’s severe. That’s also another cause for the “short-stroke” appearance of over-function: the extractor issue has slowed the carrier.

If you’re having any problems with “over-function,” solutions include retrofitting an adjustable manifold, increasing carrier mass, installing a stouter buffer spring. I do all those things on my rifles. Keep in mind that I am primarily a Service Rifle shooter, and I am trying to push an 80-grain bullet as fast as reasonably possible from a 20-inch barrel that can’t get the modifications mentioned. I know a thing or three about delaying bolt unlocking — I’ll cover more on this topic if you all want to know.

 

Sources:

Sun Devil Manufacturing

663 West 2nd Ave., Suite 16

Mesa, AZ 85210

(480) 833-9876

 

Medesha Firearms Ltd.

10326 E. Adobe Rd.

Mesa, AZ 85207

(480) 986-5876

 

Reloaders Corner: Pressure Curves for Semi-Automatic and Bolt-Action Rifles

The following is a specially-adapted excerpt from the forthcoming book, Top-Grade Ammo, by author Glen Zediker, owner of Zediker Publishing.

When you’re handloading for semi-automatic rifles and bolt-action rifles, it’s helpful to realize they are not to be approached the same way. Continue reading Reloaders Corner: Pressure Curves for Semi-Automatic and Bolt-Action Rifles

Guntec Key-Mod Handrails

Affordable Key-Mod Rails for the AR-15

Since 1989 GuntecUSA has been providing both the professional and average shooter quality products to improve their shooting. Now comes the manufacturer’s AR-15-ready, line of ultra lightweight thin key mod free floating handguard with a monolithic top rail. The all new handguards features a T6 Aluminum body. Guntec’s free floating handguard includes a proprietary steel barrel nut. The rail system slides over the barrel nut and locks up against the receiver with just six screws. Guntec Key-Mod Handrails come in the original Tactical Black, and Dark Earth, and in a variety of lengths and designs.

What makes a Key-Mod rail so great? It’s super-light-weight, endlessly customizable, and easy to use. The open sourced design allows you to place the extra rail anywhere on the handguard, giving you the freedom to choose what components to add, as well as reducing weight of the rail by getting rid of the unused rail space.

Not only is the handrail easy to install, but the key-mod rails are even easier. You never have to worry about rail covers, or hurt fingers, again. Just simply remove your accessory, then remove the rail, and you’re back to the smooth grip of the key-mod rail itself. Still unsure of how the key-mod system works? Check out the easy-to-follow equation below.


We had a quick word with our own Matt D. to get his thoughts on the GuntecUSA Key-Mod system. Matt has a wide range of experience with AR builds. Take a look at the clip below to see just how the barrel nut is set up, and how you can install it without marring your rail.
As you can see from the short clip, the design of the barrel nut makes it impossible to use most armourers wrenches. We’ve seen the results, and it’s not pretty, typically destroying the threads for the barrel screws. We used a crescent wrench, set to the appropriate width of 18mm-33mm. Each handrail also comes with shims, to ensure a super-tight fit.