Building The 90 Grain AR-15, Part One

An overview of building competition/precision .223/5.56 AR-15 to handle the heavy bullet, long-range loads.

by Joe Carlos

Two breakthroughs allowed the M16/AR-15 to replace the M14/M1A as the Service Rifle match winner in recent years. They were the float tube, which prevents applying pressures to the barrel, and high quality, accurate match bullets that are more wind resistant than 55 grain GI ball. The float tube is still with us but when is the last time you saw anyone win with 55 or even 69 grain bullets out to 600 yards? Just as heavier bullets with higher ballistic coefficients replaced the 69 grainers, we are seeing the new 90 grain projectiles making their presence known in Service Rifle competition. It’s possible a customer is going to approach you regarding converting to use heavier bullets. This series will help you provide guidance to them.

Very Low Drag .22 caliber, 90 grain bullets have been around for about a decade. JLK was the first to introduce them around 2004 and their bullet is still available through Swampworks (swampworks.com), having the highest B.C. of any 90 on the market. Berger also offers a VLD 90. Many High Power shooters avoid the high maintenance VLD bullets in any weight and don’t want the extra work of “chasing the throat.” Sierra introduced their 90 grain boat tail in 2005 proclaiming that it was a major advancement for long range shooting with the AR-15. “Long range” to most of us means 600 to 1,000 yards.

As you are probably aware, boat tails don’t normally require chasing the throat and are relatively maintenance free. The AR-15 is most commonly equipped with a 20” barrel although some “funny gun”match rifles have 26” tubes. Just as people load their 80s to different velocities, folks experimenting with 90s also push the speed envelope to varying degrees, so it can be difficult to compare the two. Using ballistic programs, I’ve come up with an estimate of about 17% greater “wind proofing” by switching from 80s to 90 grain boat tails. Going all the way to the VLD design shows an improvement of 28.6%. No doubt about it, 90s are a better mouse trap!

Saying that 90s will be about 17% more “wind proof” than 80s is a bit abstract, so I dug out a ballistic calculation program and arrived at the following example. Shooting an 80 in a 10 mile per hour full value cross wind at 600 yards results in about 34.5” of wind drift, requiring a 23 click (with ¼ minute of angle sights) correction. A 90 in the same conditions drifts only 29.5”, requiring three less clicks. That in itself is no big deal and anyone can reach up and move the sights three more clicks. If the wind never changed, life would be good, however, if there is any such thing as a range where the wind blows consistently during the twenty minutes of record fire at 600 yards I have seldom experienced such.

Where the 90s pay for themselves is when the wind is changing rapidly and unpredictably. Let’s say you have a perfect rifle capable of shooting an X every time at 600 and that you are a skilled enough to point it there. Now let’s say that your last shot was a pinwheel X and, as you admire it in the spotting scope, you see no change in conditions. What happens if, as you come off the scope and roll over to execute the next shot, the wind picks up to from 10 to 12 mph and you don’t catch it? If you are shooting an 80 that shot is probably going to come up in the 9 ring. A 90 would have held the 10 ring.

Let’s take a look at how to build these rifles.

Read more in our April 2014 issue. Back issues are available.

Don’t miss a single issue. Subscribe now or renew your subscription.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: