6.8SPC FAQ

(last update $Date: 2009/01/10 22:47:32 $)


This document is meant to answer the most common questions about 6.8SPC in AR15 rifles.

Background

I'm one of the early "regular guy" adopters of this caliber- I've had my 6.8SPC AR15 upper since March 26th 2004. At that time, I also secured two cases of the Remington pre-production ammution, which was intended for function testing only. After shooting some of that and pulling rest down, I have been shooting my own handloaded ammunition.

Pictures

  1. Link to pictures, 6.8SPC rifle
  2. Link to pictures, sectioned 6.8SPC case

The information here is correct to the best of my knowledge; either I know it first-hand ("[1]"), second-hand from someone who is in a position to know ("[2]"), or it has been publically stated and I haven't seen any contradictory information ("[PNC]").

History

The following article is the best history of 6.8 SPC to date:
The 6.8 SPC, Is it all that?   Shotgun News 60(21):4-16. July 31, 2006 [5.3 MB PDF]

6.8SPC was developed for better terminal effects while still having an acceptable long range trajectory. Anyone wanting more information should read DocGKR's post in this thread: Tactical Forums :: The Terminal Effects Forum :: 6.8x43mm SPC Rifles [PNC].

What is the point of 6.8SPC?

The point of 6.8SPC is: significantly better terminal ballistics against humans and animals to 300 yards and still fit and run reliably in the AR15/M16/M4 platform.

Is the 6.8SPC case a necked up .223? What is the parent case?

The parent case is .30 Remington, shortened to fit the AR15 magazine OAL.[PNC] This is slightly larger in diameter than .223, so a new bolt is required. Magazine capacity is slightly reduced vs. 5.56 in the proper mags.[1]

External Ballistics, Basics

When factory loads are fully developed, an 18" barrel should shoot the 115gr OTM at about 2750fps; the 16" barrel at about 2650fps; and a 12" barrel at just over 2500fps.
The BC of the 115gr Hornady is 0.340.[2] The BC of the 110gr VMAX is 0.362.[1]
>>  Reference Ballistics, Light & Fast 5.56mm
 _BC_ _MV_         0     100     200     300     400     500     600     700     800 | YARDS
0.324 3100 >   -2.55    0.00   -1.85   -9.24  -23.35  -46.26  -79.78 -127.75 -193.01 | > IMI M855 62gr
                       -0.00    1.00    3.00    5.50    8.75   12.75   17.50   23.00 Qt.MOA
                3097    2802    2525    2262    2019    1790    1582    1397    1242 | fps, velocity

0.260 3215 >   -2.55    0.00   -1.75   -9.20  -23.98  -48.96  -87.15 -144.30 -225.10 | > SA M193 55gr
                       -0.00    0.75    3.00    5.75    9.25   13.75   19.75   26.75 Qt.MOA
                3211    2838    2492    2170    1878    1608    1379    1195    1066 | fps, velocity

>>  Reference Ballistics, Heavy & Slow 223/308
0.395 2670 >   -2.54   -0.00   -3.24  -13.54  -32.14  -61.26 -102.56 -159.81 -235.18 | > 223REM Black Hills 75gr 20", chrono'd
                        0.00    1.50    4.25    7.75   11.75   16.25   21.75   28.00 Qt.MOA
                2667    2446    2235    2034    1846    1667    1506    1360    1237 | fps, velocity

0.447 2575 >   -2.54   -0.00   -3.56  -14.43  -33.69  -63.38 -104.82 -161.35 -234.58 | > 308WIN FGMM 168gr from AR10 20", chrono'd
                        0.00    1.75    4.50    8.00   12.00   16.75   22.00   28.00 Qt.MOA
                2573    2381    2197    2021    1855    1696    1550    1416    1298 | fps, velocity

>>  6.8SPC Ballistics
0.370 2660 >   -2.54   -0.00   -3.36  -14.03  -33.39  -63.96 -107.72 -168.94 -250.23 | > 6.8SPC 110gr VMAX 18", chrono'd
                        0.00    1.50    4.50    8.00   12.25   17.25   23.00   29.75 Qt.MOA
                2657    2422    2199    1987    1789    1603    1439    1293    1176 | fps, velocity

Does the muzzle velocity really meet Remington's advertised 2800fps?

The batch of Remington 115gr SMK "green box" which emerged 8-9/2005 chronos at 2460fps from an 18" barrel. My reloads shoot a 110gr VMAX at 2640fps.

How does 6.8SPC's trajectory compare to caliber X?

Since the ballistic coefficient (BC) and muzzle velocity (MV) of 6.8SPC is very similar to 75-77gr 5.56mm and 150gr .308, the trajectories will be almost identical. For other comparisons, plug the MV and BC into the ballistic computer of your choice. [2]

How can I shoot 6.8SPC out of an AR15?

The barrel and bolt are specific to 6.8SPC.[1] Magazine bodies and followers designed to work with 6.8SPC are required. [2]. Springs from a 30-round 5.56 USGI magazine can be used in the 25 round 6.8 mags. Those same springs cut down to about 14 coils work in the 15 round mags. The upper receiver is standard with the possible addition of larger M4 feedramp cuts (can be cut)[1]. A standard complete AR15 lower is used [1].

Do AR15/M16 magazines work?

No (*see note below) Not USGI 30's, 20's, Thermolds, nor Orlites. Approximately 5-6 will fit and feed properly. Loading more will cause the mag to swell and it won't fit in the mag well.[1] See this thread on Lightfighter.net for more information: Lightfighter Tactical :: Lightfighter Tactical Forum :: Weapon Systems :: 6.8 SPC Experiments.

Where can I get loaded ammunition

October 2005, Silver State Armory is currently shipping 6.8SPC ammunition using their own high-quality brass.
As of July 2005, Remington factory ammunition has hit distributors and has shown up at online vendors and local shops. Precision Crafted Ammunition is also currently shipping 6.8SPC ammunition.

Proper 6.8SPC magazines are available from several sources:

  • PRI directly in 5, 10, 15, and 25 round capacities. The 15 round mags may hold up to 18, but feed best when loaded to only 15. The 25 round mags may hold up to 28, but feed bet when loaded to only 25.
  • MSTN also sells the complete PRI mags.
  • Barrett is supposed to be selling the 15 and 25 round magazines also.
  • Conversion kits consisting of a new magazine body and replacement follower are available from MSTN to convert your old pre-ban AR15 5.56mm magazines to be 6.8-compatible, also in the 15 and 25-round capacities. This works just like Armalite's deal for AR10 conversion kits.[1]

* - There is one report that the cheap 20-round Triple-K 7.62x39 magazines work with 6.8SPC.

Magazines springs Springs for USGI 5.56 30-round mags can be used in the 25 round PRI mags. Cut these springs to approximately 14 coils for use in the 15 round PRI mags.

Where can I get a complete upper in 6.8SPC

MSTN is shipping uppers.[1] Model 1 Sales is advertising 6.8SPC uppers. Barrett Rifles is also advertising 6.8SPC rifles.[PNC]. AMEETEC Arms is advertising 6.8 uppers. DPMS lists 6.8 uppers.

What are the muzzle attachment dimensions?

To avoid unpleasantness if a 5.56mm muzzle attachment were screwed onto a 6.8SPC barrel, a different thread pattern is used: 5/8"x24tpi. This is the same as the Remington 700.[1] This is the thread pitch specified by the military.

It has been reported that some other 6.8 barrels are using a 1/2-32 or -36 thread pitch. I have no other information at this time.

What muzzle brakes / flash-hiders are available for 6.8SPC (5/8-24)

  1. The MSTN/PRI Quiet Control ("QC") Comp. This is a modern comp and should be as effective as the JP BC Comp, the Miculek comp, or the Benny Hill comp (which are only available in 5.56).
  2. The Vortex flash-hider. Smith Enterprises' Vortex for the Remington 700 PSS (.308) will work with 6.8SPC. You must request some custom machine work to match the shoulder-muzzle dimensions. It is threaded for 5/8"-24tpi.
  3. Phantom 308 flash hider.
  4. OPSINC 4-port brake. Pictured above, this is an older-technology brake.
  5. OPSINC 2-port brake. This brake has only one chamber and is primarily designed as an attachment point for an OPSINC suppressor. OPSINC suppressors are only available to LEO & Military in the US because OPS is located in California.
  6. Armalite AR-10 muzzle brake.

What is recoil like?

Shooting an 18" 6.8SPC barrel with the OPSINC 4-port muzzle brake has slightly more recoil impulse than a 18" 5.56mm with no brake. It is more noticeable than 5.56 recoil in rapid fire (e.g. double tap, hammer) and full-auto, but much more controllable than 308 weapons with comparable rates of fire. [1]

Are reloading dies available? If so, from whom?

Hornady will make you a set of custom 6.8SPC dies. $90 for a standard set, or $109 for a match-grade set (with bushing-style resize die). Turnaround time is 6-8 weeks.[1] Call them if you are interested.[2]

Redding has their "Series B" dies available for 6.8SPC, from distributors for $40-45.

Is new brass for reloading available?

MidwayUSA is selling new Remington 6.8SPC brass in 200, 500, and 1000 case lots.

What bullets are available for reloading?

The bore diameter of 6.8SPC is .277", the same as the popular .270 Winchester. Since the maximum cartridge OAL is pretty short and case capacity is limited, short bullets are preferred. The likely weight range of bullets is 90gr - 120gr. The 110gr Hornady VMAX shoots really well.

Barnes has just announced a new 6.8SPC 110gr Triple-X bullet.

There are currently no 115gr bullets available commercially. A Hornady and/or Sierra 115gr OTM may be available in the next 4 months.[2]

What bullets are appropriate for hunting deer?

The 110gr VMAX is too lightly constructed for deer, even at slow velocities. I recommend the following as good choices for deer: 110gr Sierra Pro-Hunter, 100gr Barnes X, 120gr Barnes X, 110gr Barnes Triple-X. There have been some reports that the 115gr OTM performs well, but this was not designed as a hunting bullets.

What bullets are best for self-defense?

(1) 110gr Hornady VMAX, (2) 115gr Hornady OTM

What bullets are best for target shooting?

(1) 115gr Sierra OTM first, (2) the 110gr Hornady VMAX second.

Where can I find reloading data

Reload data is currently available from both Hodgdon and IMI:
  1. Hodgdon 6.8SPC data
  2. IMR 6.8SPC data

Zak's Reloading Information

I have made available some 6.8SPC reloading information here: Zak's 6.8SPC reload data.

I'd like to acknowledge the help of Marty ter Weeme in simulating the internal ballistics of some of these loads, in addition to Derek F. who connected interested parties.

What about cartridge over all length (OAL)?

The whole point of 6.8SPC is that it feed through existing AR15 lowers and as such, the cartridge over-all length is limited. This in turn limits the bullets which will work to those whose ogive is blunt enough to be held by the case neck and still fit the OAL requirement.

The PRI magazines accept rounds loaded out to 2.315". I'm loading at 2.300".

Can I load 6.8SPC on my single-stage press?

Yes. Get yourself a set of dies from Hornady, and use a .30Remington shell holder (RCBS #9219).

Can I load 6.8SPC on my Dillon press?

Get a toolhead, populate it with your 6.8SPC dies. The following shellplates work: 40SW, 10mm, 30REM. I am using a 308 casefeed adapter with a piece of duct tape partially over the plastic feed bushing's opening for the shorter 6.8 case. Use the large primer system.

[Zak Smith] [zak@computer.org] [/~zak/firearms/6.8SPC/faq.php]
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