There are several reasons why someone might want a long gun and a handgun in the same caliber. The most often cited reason I come across is compatibility of magazines; the ability to use the same magazines to feed the handgun and the long gun. The down side of this is that you are restricting your long gun to a handgun caliber (the one arguable exception being the 5.7mm pairing). The benefit to this strategy is that you can have an effective handgun and a (relatively) effective long gun and if you have spare magazines for either you have spare magazines for both. In this case we’re talking about the Just Right Carbine in 9mm that uses magazines for the Glock Model 17.
Standard Glock 17 magazines hold 17 rounds of 9mm ammo. Yes, you can get +2 floor plates for the magazines giving them an increased capacity of 19 rounds. Yes, you can buy factory and after-market magazines for the Glock 17 that hold as many as 30 or 33 rounds. While I’m not a fan of using 30+ round magazines in my handgun, having them available for my long gun would be preferred and being able to use them in either is a desirable versatility (in my opinion).
For my testing purposes I had on hand a fourth generation Glock Model 17, a Just Right Carbine (JRC) designed to use the magazines of the Glock 17, and eight magazines for said Glock 17. Prior to any cross-magazine testing I did some research. On the Just Right Carbine webpage you can find some informational PDFs that discuss potential malfunctions and their causes. The one that most caught my eye was apparently caused by seating the magazine into the JRC too far. Apparently, if you slap the Glock magazine into the JRC long gun, it’s possible to seat the magazine too far in and thereby create potential feeding malfunctions. The published information from JRC indicates that you should push the magazine in until it clicks in place and then pull on it to insure that it’s positioned properly.
My challenge and concern with this is that the JRC is obviously marketed to the law enforcement community and some agencies will undoubtedly purchase it for issue as a “patrol rifle.” (For the sake of understanding, if the long gun is chambered for a handgun caliber, I don’t consider it a rifle. I like that it’s the Just Right CARBINE because that term is more appropriate given its chambering.) Police officers and deputies, like soldiers, are taught a simple and specific method for clearing malfunctions in a hurry – because they’re training to use these weapons in high risk circumstance. The very first step in clearing the malfunction is to SLAP the bottom of the seated magazine to insure that it’s fully in place. Doing so with a Glock magazine in the JRC will only make a potential misfeed situation worse.
That means that extra training and familiarity with the JRC is required, and if there’s any chance that the shooter is using both a regular AR chambered for 5.56mm or .223 AND a JRC in 9mm, then a conflict will be created in malfunction drill training. That needs to be kept in mind, planned and budgeted for. Above all else, no officer or deputy should be put in a position where his (or her) training to clear a malfunction is SLAP RACK TAP SHOOT and then they’re issued a JRC where such malfunction drill won’t clear the potential malfunction.
Outside of that concern, the Glock magazines seem to feed and function just fine in the JRC. Using live ammo and dummy rounds I was able to load and feed my test JRC with no issues. I intentionally did slap several magazines of ammo into the weapon in an attempt to cause the feed malfunction but was unsuccessful. I fed both ball and semi-jacketed hollow point ammo into the JRC with no issues.
The JRC is specifically designed so that it can be set up for a left- or right-handed shooter. Mine was delivered with no sights: just flat top picatinny rail, and for the purposes of my testing I attached an EOTech site. Reported accuracy for JRC carbines is acceptable for a contemporary weapon system and is (obviously) dependent on the shooter and ammo used. I use Speer Gold Dot ammo in my Glock 17 and the JRC test carbine I have fed that with no problems.