Observations on Chamber Inserts and Subsonic Rifle Loads

© 2014 – Ed Harris

In my testing the Sportsman’s Guide inserts for the .303 British or 7.62x54R worked well. The ones they offer for .308 Winchester and .30-‘06 had “issues” which will I will explain later.  I currently have adapters for the .303 and 7.62x4R to use  .32 ACP and .32 S&W Long/H&R Mag., ammo. The .32 ACP when fired in a rifle adapter is very quiet and shoots close to point of aim at 50 feet with the battle sight set to 300 yards or meters. The .32 H&R Magnum insert is louder, but more accurate, but it also works with .32 S&W Long ammo, which is quiet like the .32 ACP. These adapters could be worthwhile to include in your kit if you carried a handgun in the same caliber, or fooled with the adapters enough to confirm good sight dope, to use for them small game, to conserve your big game rounds.

CAUTION is required when attempting to use these adapters in rimless calibers! Neither the .308 Win. nor .30-'06 adapters from Sportsman's Guide would fit into a Wilson Max. Cartridge gage and both required modification to adjust dimensions which were "off".  Even so, I succeeded in "sticking" a .30-06 adapter in the chamber of a 03A3 Springfield.  I was able to get it out by carefully forcing the bolt closed over it, then inserting a narrow screwdriver blade between the extractor and receiver ring to prevent the extractor from hopping off the rim until I could open the bolt with gingerly applied blows of a dead-blow lead hammer. Had the rifle in which I had tried the adapter been a "slam-feeder" with plunger ejector, such as like an M1 Garand or Remington 700 it would have required gunsmith removal of the barrel to get it out!

These adapters have recently been offered which use 7.62x25 ammunition.  I would not recommend this because the 7.62x25 Tokarev cartridge is loaded to about 40,000 psi vs. 20,000 for the .32 ACP.  If you fired a lot of these in the steel adapter it will expand to tightly conform to the rifle chamber.  If your chamber is smooth and round you might get away with it, but if it is rough or out of round in places, you may "stick" the adapter in the rifle chamber. Re-read the paragraph above!

These adapters work best in the .303 British and 7.62x54R RIMMED cartridges, in military surplus rifles which have fairly sloppy, "forgiving" tolerances which ease extraction of the adapter. Typical accuracy is about an inch at 50 feet with iron sights.  Don’t expect any better.

If you hand load, making your own reduced power small game loads for your center-fire rifle makes more sense. Back in the day special ops units used to improvise suppressor rounds by pulling the bullet and dumping the pistol powder charge from a .45 ACP hardball round into a 7.62 NATO case with a standard M80 Ball bullet.  When stateside ballisticians found out what the troops were doing, they provided a Bonanza bullet puller, and a Lee dipper to throw about 5-1/2 grs. of Bullseye or 6-1/2 grs. of Unique to do the same thing. (You need to increase the charge 1 grain in the .30-'06 or 8mm Mauser). Reduced loads with boat tail FMJ M80 bullets are stable in a 10 inch twist, but accuracy may be poor at subsonic velocity in a 12 inch twist unless the M80 bullet is loaded BACKWARDS! Seat and crimp in the cannelure, with the boat tail forward rounds will feed from the M14 magazine. The .30 M1 carbine 110 FMJ or Cal. .30 Ball M2 flatbased bullets also work OK for these loads.

These subsonic loads do not function the M1, M14 or M21 so you must manually cycle the op rod. Lubricated lead bullets are best for reduced loads, and generally exit a rifle barrel reliably when loaded above 800 f.p.s.  Ballistic uniformity is important here, because erratic velocities can provide unpleasant surprises which range from bullets stuck in barrels to blown up rifles!  Fast burning pistol powders such as used in the .45 ACP and 12-ga. shotgun powders used for light field and target loads work best. Reduced loads with jacketed bullets are much more problematic and must be kept above 900 f.p.s. to e sure reliable bore exit unless you take precautions. The barrel must be clean and should be LIGHTLY oiled then wiped with one dry patch to avoid sticking a bullet in the bore. OR increase the charge a grain or so to adjust velocity high enough to ensure reliable bore-exit, but still subsonic for low noise (1050 +/- 30 fps). Better to use a well lubricated cast lead bullet which has less bore drag.

If you experiment enough with subsonic reduced loads in rifles you WILL stick a bullet in the bore eventually. The safe way to remove this is with a .32 revolver SQUIBB ROD from Brownells. This is a solid brass rod about 8 inches long which threads onto a Dewey rifle rod using an adapter. The .32 cal. revolver squib rod works in the .303 British and 7.62x39 or .303 British rifles as-is. You may need to reduce its diameter slightly by spinning the rod in an electric drill and polishing with emery cloth and light oil to get a sliding fit in tight, unworn .30 cal. bores.

To remove your stuck bullet, extract the fired case, open the action and remove any remaining cartridges. Use an eyedropper to deposit a few ccs of penetrating oil, such as Kano Kroil or VV-L-800 down the muzzle, and stand the rifle in a corner for 15 minutes to let it soak while you contemplate your sin. While the oil is working, thread the squib rod onto your cleaning rod. Then after 15 minutes of rapt contemplation insert the rod with squib rod attached down the muzzle and start tapping the bullet out with a multitude of many LIGHT taps. DO NOT use a hammer. Patience is a virtue. It helps to focus your thoughts on your stupidity, learn from your experience, go forth and sin no more... Whatever works, if you are not spiritual recite your General Orders, sing the Marine Corps Hymn, ponder Maxwell's equations....

Better for such use today is IMR Trail Boss which is a bulky, fast-burning powder originally developed to replace black powder for maritime harpoon or line throwing guns, and as the boosting charge for 40mm grenades used in the M203.  It bulks up well in the case and is well suited for reduced loads in rifle calibers. About 6 grains of Trail Boss in the 7.62 NATO or 7 grs. in the .30-'06 will exit the barrel reliably using the Speer 110-gr. Varminter JHP bullet, which works well at subsonic velocity.

Further info at

Subsonic 308 Thread at The Firearms Forum


Trail Boss 308 Loads Thread at The Firearms Forum


IMR Trail Boss Reduced Loads – Data File (.pdf)


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