I recently built two .22 LR pistols, Both using CMMG/RTB BCG and barrels, one a 4.5" and one a 9".  Both used Canto Arms barrel nut and barrel nut extension.  Both used 25 round Black Dog magazines.

On the 4.5 inch I used a Canto Arms receiver extension plug and on the 9 inch I used the Spikes Tactical RE plug.

The Spikes Tactical plug does not have a hole drilled all the way through and uses a raised hex-nut shape machined into the rear face of the plug for tightening.  The Spikes does not have the receiver end plate notch so you either must grind off the nub or use a receiver end plate that does not have that nub.  Or don't use one at all if you have made your take down detent captive.
The Spikes stops about 1/4 inch short of the back of the BCG in the upper, and therefore, does not put any pressure on the back of the .22 LR BCG.  
Spikes incorporates a freely rotating QD socket in the back of the plug. There is no knurling on the sides of the plug since you use a wrench to tighten down the plug.  Price, about $19 from Brownells.  That does not include a QD swivel.

Now the Canto Arms.
The Canto Arms plug is flat across the back face and you use a hex wrench inserted into the QD socket to tighten the plug in place.  It does have knurling on the edge of the plug, which looks nice, makes it easier to tighten down, but you still must use a hex wrench to finish tightening.  The CA plug extends all the way into the receiver extension boss on the lower and DOES put pressure on the back of the BCG.  The CA unit also has the receiver end plate notch cut in it so you can use any receiver end plate you want, but you MUST use the receiver end plate.  If you do not, the plug will screw in too far and make contact with the upper as you try to close it.
As with the Spikes plug, the CA unit has a freely rotating QD socket in the rear of the plug.  The hex wrench socket DOES extend all the way through the plug so if you do not use a QD swivel in the hole, it leaves a place where dirt can enter or more importantly, for gas can escape and that's going to come straight back into your face if a case were to rupture.
Price, $20 from Canto Arms. That price does not include the QD swivel ($6 from CA).


Which one should you use?  That's up to you really but I think the looks of the Canto Arms is better than the Spikes and I like the way it puts pressure against the back of the bolt carrier group. Some have glued silicon pads on the face of the Spikes to put pressure on the back of the BCG but that's something that does not sit well with me.  Just one more thing to come loose and possibly gum up the works, not that it really matters on a plinker.  But I do know I'll be replacing the Spikes with another Canto Arms unit.

Now to Black Dog magazines.  Function wise, both feeding, loading, and ejection, these mags are 100 percent and could not work better, with HP, RN, and FN .22LR bullets, no subs were used.

 But what about dropping free?  Out of 4 lowers I tried, three different brands, the four BD magazines I had would not drop free in all of them.  I had to take the tightest lower of the lot and start fitting the mags so they would drop free.  I had to sand the dickens out of the sides of the mags (sonic welded magazines) and the front and back of the two mags that were bolted together.  Measurements indicated that there was as much as a .015 difference between the mags in various areas, so the Aero lower, two NFA polymer lowers, and an Anderson lower (all drop free with Lancer and OKay magazines) would not drop all the BD magazines.  After sanding down the offending areas with 150, 320, 600 grit wet/dry, they all drop free from all the lowers now, but should one really have to put all the elbow grease into making a $19 magazine drop free?  That's up to you.  Personally, I think they are worth the effort and after finishing with 600 grit and hitting with a CLP cloth, they look like they have never been touched.  


Just a few minutes ago I received two Black Dog 15 Round magazines, the sonic welded units.  Both of these magazines dropped free from all four lowers.