The explosion of calibers available for the AR15 platform over the last 15 years has just been amazing. These options try to address the specific needs of the user to include: 3 gun, varmint hunter, cross the course, tactical, and just the ‘more power’ crowd. Here is an overview of the calibers I have either worked with or explored (ie. not every caliber that has ever been offered).
L-R: 223/5.56, 6.5 Gendel,6.8 SPC, 300 Blackout, 7.62×40 Wilson, 458 Socom
- 204 Ruger – Bullets are too small for me to pick up! I don’t see the advantage of this caliber over a regular 223 loaded with lighter bullets? Just not enough interest for me to give it a try.
- 223 Rem – The original. With all the work done by the manufactures, hard to beat for the casual shooter. The choice of barrels/contours/twists should meet any need. Loaded ammunition is widely available, again with a mind numbing array of choices. For the casual user, make sure what you gun is chambered to handle: 223 rem,223 wilde, or 5.56 Each has an advantage depending on your use.
- 6×45 (6mm/223) – This was the first wildcat I remember people adapting to the AR 15 platform. It is simply the 223 Remington case necked up to accept a 243/6mm bullet. At the time the 223 was still using the slower twist with 55gr bullets. So the 6mm offered a selection of heavier bullets, delivering better ballistic performance. In recent times, this case has been overshadowed as bullet manufacturers developed 69gr and even 77gr bullets that could be loaded magazine length in the 223.
- 6 mm Hagar – A recent design to meet the needs of cross the course shooters. Based on the 30 Rem/6.8 SPC case. See Creedmoor Sports for more information. The advantage of this design is the added case volume vs the 6×45. Negatives include: switching to a 6.8 bolt and magazines, can only handle up to 87gr bullets at magazine length, cases cannot be formed from any readily available case.
- 6.5 Gendel (I will include in this the variations to avoid the Alexander Arms copyright – 6.5 LBC , 6.5 CSS, etc). Based on the 220 Russian case that has been used for many other wildcat rounds through the years. This cartridge again requires a special bolt and magazine. A whole host of issues have slowed the adoption of this cartridge by rifle manufactures. Will not list them all here. While still experimenting with this round, I have not been able to reach the advertised performance.
- 6.8 SPC – Another case with a long and windy road. There was a dip in interest for a while, but lately I see a renewal in manufactures offering finished guns and barrels for custom builds. I have had several configuration in this caliber. All have performed well. Accuracy is on par with the 223, with a noticeable increase in power. However, as with all AR15 mag. length cartridges, powder capacity is an issue. I see no reason for barrels longer than 20 inches. You just can’t get enough powder in the case to make use of the extra barrel length. Of course it requires a switch to the bolt and magazine from a regular 223. The parent case (30 Remington) is no longer produced, so you are forced to buy brass.
- 300 AAC/Blackout – A great development based on the 223 case. No need for a new bolt or magazine. Designed to shoot sub sonic or super sonic loads. More power than the 233, but not quite 6.8 SPC. Nice bullet selection, with companies introducing new designs specifically for the round. You can form your cases from regular 223 brass, so it can be easy to get started loading your own ammo. Uses the standard 223 bolt and magazine. I have heard some mentions that accuracy has been an issue, but I have yet to experience anything out of the norm (easily under 1 MOA)
- 7.62×40 Wilson – A development by Wilson Combat. Another take on the 308 bullet in a 223 case. Like the Blackout, can use the existing 223 AR bolt. But they recommend using their modified magazines to get a full 20 rounds in a 20 round magazine. The finished case length is longer than the Blackout, but can be made from regular 223 brass. Their design is not intended for sub-sonic use. Nice increase in power from the Blackout.
- American 30 BHW – Relatively new design from Black Hole Weaponry. This is a 308 caliber based on the 6.8 SPC case, giving it a lager case volume than either the 300 Blackout or the 7.62×40 Wilson. Their posted ballistics are really impressive. Barrels just becoming available.
- 338 Spectre – Another development from Teppo Jutsu (see 458 Socom). Designed as a sub sonic round. Low pressures mean you can use most 9mm suppressors. Have yet to try this round out, but there is always tomorrow!
- 450 Bushmaster – An interesting idea on the large caliber AR offering. But this came a little late to market. There were other options (see below) that had sizable advantages. What I think hurt this case most was the limited bullet selection.
- 458 Socom – One of the first viable options for a large caliber AR 15 platform rifle. Developed by Teppo Jutsu, this design uses the much more popular 458 bullet diameter. Bullets go from 250gr to 600gr! I have found the 325gr Hornady flex tips to be incredibly accurate. Suppressor manufactures have begun to support this caliber, providing a whole new avenue to explore. While it does need a special bolt, you can use your standard MagPul magazines.
- 50 Beowulf – Love the name, but that is it. Initially handicapped by Alexander Arms control over manufacturing. Limited suppliers of brass and few options for bullets. Not enough to choose this over the 458 Socom.
Hatcher's Notebook 