A Glock 20 10mm factory barrel actually will function in a G21/G21SF slide and maintain good accuracy. This has been clearly proven in live-fire testing, the results of which are available in multiple cases, for example, Post #79 of the Glock Talk thread titled "Glock 21 conversion to Glock 20?" posted in the General Glocking Forum (Update: the thread has been retired). The O.D. of a G21 and G20 barrel are identical and the barrel lugs are compatible. However, while the G20 barrel is a good fit in the other dimensions, the 10mm factory barrel chamber hood is narrower than the 45ACP barrel hood by slightly more than .050" and this leaves the chamber to breechface interface undersized. The dimensions of a 45-10mm conversion barrel are identical to a G20 10mm factory barrel except that the barrel hood is made .050" wider to better fit the 45ACP pistol breechface.
Although a stock G20 10mm barrel will function in the G21 slide, the recommended conversion of the G21/21SF .45ACP pistol to 10mm caliber strongly favors an aftermarket 45-10mm conversion barrel for best results. An aftermarket barrel typically has better chamber support than the Glock 20 stock 10mm barrel, thus the brass casings are less likely to suffer damage which is critically important if you reload them. The conversion barrel has conventional rifling (land and groove) suitable for non-jacketed (hardcast lead) bullets of all weights for 10mm. In addition, the aftermarket barrels are available in extended lengths which provide greater velocity and energy. For all these reasons, most shooters want this capability to take full advantage of the potential of the 10mm caliber in their converted pistol.
A drop-in 10mm barrel (aftermarket conversion barrel strongly recommended) and G20 mag change are the minimum requirements to shoot 10mm caliber ammo with the G21/G21SF pistol.
Many owners find that the stock .45 extractor will function reliably to extract the 10mm case in their converted pistol. A G21 pistol that happens to have a looser fit in the distance between extractor and bore axis can experience erratic ejection and occasional failure-to-extract problems. Replacing the .45 extractor with a G20 10mm factory LCI extractor (Part No. SP 01909) is an alternative, but it requires more time and effort to convert the pistol from 45ACP to 10mm and return it back to 45ACP service. There is another option suggested by KKM Precision, Inc.
Barrel manufacturer KKM recommends some minor gunsmith work on the stock .45 extractor that enables it to function more reliably for 10mm and continue to work for 45ACP. It involves removing no more than 0.020" from the 'fitting pad' on the inside of the .45 LCI extractor (see photo below) allowing it to travel closer to the axis of the bore and make solid contact with the 10mm case. As material is filed from the fitting pad, test for proper tensioning of the extractor against a cartridge. With the slide removed from the frame, insert an empty 10mm case or dummy round underneath the extractor claw from the bottom of the slide. There should be some measurable tension being applied by the extractor’s vertical engagement edge to the cartridge. A dummy round should remain suspended by the inward force of the extractor. Some who have studied this option believe it is equivalent or better than installing a 10mm extractor. Contact KKM for advice and instructions before attempting this modification (Phone: 775-246-5444). It is best to modify a spare extractor in case of a problem.
All the Gen3 and Gen4 Glock .45 ACP and 10mm pistols use the same ejector (marked 8196-2) so that is never an issue for the conversion.
Conversion barrels in both stock length (4.60") and extended length (5.30" & 6.02") are available from Storm Lake. They cost approximately $160-$180 for stock length depending on the desired finish (Stainless or Black Isonite QPQ™). IGB Austria offers 10mm barrels for the G21 up to 7.5 inches long which can be purchased in the USA. IGB 10mm barrels are more expensive at $220 for stock length. These aftermarket conversion barrels have a fully supported chamber; important for firing full-power 10mm factory loads with minimal risk of damage to the brass casing. The rifling in the conversion barrel is suitable for firing non-jacketed, hardcast lead bullets of all weights for 10mm.
Another conversion option is to purchase a complete OEM G20 upper assembly (slide with 10mm factory barrel and internals) and install it on the G21 frame.
For reference, a complete OEM G20 upper retails for $360 from Glockmeister
and wait times are long. The complete upper from Glockmeister includes a Glock factory 10mm barrel which is a waste of money if the shooter needs better chamber support for hot ammo and the ability to shoot non-jacketed (hardcast lead) bullets. Other sources of a complete, used G20 upper include the Classified Ads at GlockTalk.com
where they are often available for around $275. A Glock complete upper assembly is not a firearm under federal law and can be shipped directly to the buyer.
Some may claim that the only "safe" way to make the conversion is a full upper replacement with a G20 slide because of the slightly heavier weight of the G20 slide compared to the G21 slide.⁽¹⁾
The short answer to this unfounded concern is the added mass of the G20 slide was left there to slow the recoil reaction (unlocking) for the higher energy 10mm round and to attenuate felt recoil, but not as a structural requirement for safe operation. A difference in slide velocity between the two can be partially compensated for with a stronger recoil spring in the G21 slide.
Glock warns owners not to fire non-jacketed (hardcast lead) bullets in the factory barrel with polygonal rifling. Doing so voids the Glock warranty. Many shooters believe the chamber support of the G20 factory barrel is insufficient to prevent damage to the brass casings of full-power 10mm loads rendering them unsuitable for reloading. The factory barrel with polygonal rifling is known to have problems stabilizing hot loads of 10mm non-jacketed, hardcast lead bullets heavier than 200gr.
Greg Kinman (Hickok45
) has shown that DoubleTap 10mm 230gr WFNGC hardcast lead bullets tumble and accuracy suffers drastically fired from the factory 10mm barrel but perform well when fired from an aftermarket barrel with conventional rifling. The lighter DoubleTap 10mm 200gr WFNGC hardcast bullet performs well with the factory or aftermarket barrel. Reference Video: Glock 20 with Heavy Cast Bullets (Chapter 2)
If you plan to frequently shoot full-power 10mm loads, then you can consider something stronger than the factory standard 17-lb recoil spring in the G21, perhaps one in the 21-23 lb range. Springs are available from Wolff Gunsprings
. I prefer Wolff springs because I have greater confidence that the measured spring strength matches the rating of Wolff springs. If the converted pistol with stock recoil spring exhibits signs of frame battering or brass is thrown a great distance, try a 21-lb spring first and increase the strength only if brass is thrown further than 10 feet. Felt recoil is likely to increase with a stronger recoil spring. Examples of full-power, factory 10mm loads are 135gr at 1600 fps, 155gr at 1500 fps, 165gr at 1400 fps, 180gr at 1300 fps, 200gr at 1250 fps, 220gr at 1200 fps and similar loads.
What you most want to prevent when firing full-power 10mm loads, and the thing most likely to cause serious damage, is an occurrence of out-of-battery detonation. Keeping the chamber clean and using a more powerful recoil spring will minimize this risk.
A stronger recoil spring also yields more consistent muzzle velocity from shot to shot for a given ammo load.
The experienced 10mm shooters at Glock Talk in The 10 Ring
forum can give you advice on the appropriate guide rod and recoil spring strength
1. Photo comparison of OEM G20 10mm slide (left) and OEM G21 45ACP slide (right). The muzzle end of the slides are at the bottom of the photo. Note the milled area on both sides of the G21 slide removing non-structural mass forward of the chamber opening.