Caliber Conversions

cross-linked to my generic Calibers Reference page

There are too many different conversions to list all of them, however I wanted to show a few especially interesting ones.  Any caliber conversion could be built on any AR lower since the upper is the only part that requires modification, except in the most extreme of circumstances.



Originally a prototype by Rhineland Arms that was never completed.  Unsure if the design was sold now that Rhineland has retired.  Now produced by a new company, it uses P90 magazines on a modified AR upper.  The lower is any standard AR lower.  I believe this was the first ever AR-based P90 magazine 5.7 platform.   

Excerpt from the Rhineland AR57 promo data:

The AR57 rifle is designed for use in any climate and ready for the most extreme use you can give it. Sand and mud will normally clog and jam most rifles and carbines yet have very little effect on the AR57 due to its simplicity and strength. The AR57 is more than a challenge to any SMG, carbine or military rifle in the areas of accuracy, ergonomics, reliability and durability.

Designed as a upper that can be attached to any M16/AR15 lower, it is not dependent on the lower for reliability of ejection or accuracy. The AR57 features ambidextrous cocking handle and magazine release levers with brass ejection out the bottom of the magazine well. The barrel is free floating for improved accuracy and includes a standard M16 flash hider.


This design could be the basis for a nice little PDW / varmint / small game / plinking system.  One could probably use the magazines with a little modification for something like a .17 HMR or .22 Hornet.  I haven't researched this so it may not be possible, but it is food for thought.




The "AR57" that has recently been announced on www.57center.com and is now shipping to various retail vendors.  To be honest I like the looks of the Rhineland system a little better! 



50 Beowulf


Anecdotal reports are that the .50B is seeing some bolt failures, but I've only read of Internet posts with the issue.  Therefore, unsure as to how widespread this issue really happens to be.


6.5 Grendel

A long-range match-grade round based on bench rest target shooting rounds such as the 6mm PPC.  Essentially a 6.5 PPC with slightly higher case capacity:




S&W is producing a 5.45x39 (Russian) caliber upper.  Due to the corrosive nature of military ammunition I'm unsure as to the long-term effects on the upper.  One would assume a severely shortened life expectancy for the gas tube and possibly corrosion of the bolt.



Practically every big name company makes a 9mm upper and conversion for the AR. “Conversion” is usually a drop-in shim for the magazine well, and they usually use readily available surplus mags.


45 ACP

There are drop-in conversions and total conversions for the venerable 45:



6.5 MPC

Designed by a former United States Marine, this cartridge uses a .223 cartridge necked up for a 6.5mm.  Conversion requires ONLY a barrel replacement; all other .223 parts still work as issued.  Yes you use standard 6.5mm bullets! :)  I believe this is marginally different from a 6.5-223 round in that this round is built to fit within an AR magazine.  I am unsure of the 6.5-223 OAL so this may be irrelevant.

Its purpose is to provide greater energy out of the shorter barreled AR family rifles (M4 etc) to maintain higher performance normally expected with longer barrels out to "typical" engagement ranges.  This is purportedly a 300-yard round.  Recommendations are up to 120gr bullets.  SSK's published stats:
95 SSK Solid
- - - - - - -
2600 FPS

2800 FPS
110 Sierra HP
- - - - - - - - - - -
2480 FPS

2731 FPS
120 SMK
- - - - - - - - -
2220 FPS

2400 FPS

This 6.5 MPC is a really promising round, perhaps best described as a little brother to the 6.8SPC.  This one has not received as large an amount of media coverage because everyone hears "6.8 SPC" and gets all giddy.  There was a good write-up in one of the "Special Weapons" magazines that did a side-side comparison to other intermediate AR-framed rounds.

This round is definitely worth consideration and could give the 6.8 a real run for its money.  The ONLY thing extra that you need for this is a 6.5mm barrel - since it is built on a .223 cartridge, everything else is the same!  I am unsure if you can reload by using a mix of .223 and 6.5mm dies but you probably can.  SSK apparently makes dies as does CH4D.

It is "sponsored" by SSK, who are known for their wildcats and AR calibers such as the venerable .300 whisper - so this isn't a fringe idea.  More info here:






There are similar 6.5mm builds based on the .223 cartridge, such as the 6.5mm TCU and the 6.5-223. 




6.8 SPC

The 6.8 is designed to meet the need for a medium- to long-range cartridge that delivers effective stopping power at longer ranges where the SS-109 62gr is not as adequate.  Ballistics charts peg its effectiveness as "similar to .308 performance out to 400 yards" with the added benefit of a cartridge that fits an AR platform and maintains 5.56x45 / .223 accuracy.  Its use in the military may best be described as a "designated marksman" platform.

Dad just got an AR lower (oh no!) and now we're all talking about matching family 6.8 rifles :)

Ruger is now making a Mini-14 in this caliber!  The "Mini 6.8 ranch rifle".

More information links.  Being the hot item that it is right now, there is as much marketing data as there is actual informational links.  Possibly the best starting point for resources would be the 6.8SPC forum on http://www.AR15.com.

I have found the excellent Shotgun News article PDF write-up and posted it on my Skydrive site here!

Additional 6.8 info links:





Zak Smith's 6.8 Resource Pages


http://www.68spc.com/ (currently unavailable, but was/is the info site for ar15barrels.com and links to Zak's pages about this caliber.  Links still available via Internet Archive although some of the downloads or links may be dead.  Fortunately, most information can be gleaned from the remaining 6.8 sources.

Zak's 6.8 Specifc Pages:



5MB PDF of Zak's SGN article on the 6.8: sgn_68spc.pdf



SOF 2004 Article on the 6.8:

Scan1.JPG (162207 bytes)    Scan10001.JPG (165126 bytes)    Scan10002.JPG (149161 bytes)    Scan10004.JPG (128644 bytes)    Scan10005.JPG (146031 bytes)


A few 6.8 builds - unsure as to the source of the "field" pics. 

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