Size Does Matter! – Slugging a Surplus Rifle Barrel

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If you have any experience with surplus military firearms you know that it is often times a crap shoot as to the condition of the firearm when you get it, particularly, firearms purchased through online resources.  Many surplus firearms have seen a lot of battle usage but that doesn’t make the firearm unshootable or the proverbial wall-hanger.  However, what often can make or break the decision to keep and shoot the firearm is the condition of the bore.  How well the firearm shoots often is determined by bore condition – is it worn, is there pitting, the lands and groove condition, bore diameter and head space.  Nevertheless, a bore that is frosty from corrosive ammunition (primers) and is a little worn does not negate the firearm from an enjoyable time at the gun range, nor does it make the firearm incapable of delivering decent groups. With a little work we can improve the accuracy by simply slugging the bore to determine how much wear the bore has experienced.

Another reason to slug a surplus firearm’s bore is that not all firearms that claim to be their specific caliber actually shoot the size bullet one would suspect.  For instance, a Mosin Nagant rifle with a caliber of 7.62x54r doesn’t shoot a 308″ projectile typically loaded in 7.62 (.30 caliber) rifles.  The same principle applies to other common military firearm calibers such as 8mm Mausers, British Enfields, Arisakas, and M95 Steyrs to name a few.  Add in to this a barrel that’s been shot a few thousand times and manufactured with worn tooling parts during the war effort and any suspicion as to the size of the needed projectile to reload is completely guess at best; unless you slug the bore.  It’s even more important to determine the bore diameter of your rifle if you choose to shoot cast lead bullets.  To gain the best accuracy out of your firearm you need to slug the bore otherwise accuracy will suffer, and determining the actual bore diameter will help eliminate leading issues with cast bullets.  So, here’s how I slug a bore to determine my optimum bullet size needed for my firearms.

Before we begin we need to obtain a few inexpensive items to slug a bore.  As shown in the photo above, we need the following tools and materials: a hard wooden dowel (smaller than your bore you are slugging, preferably oak), grease, soft lead sinkers or lead slugs, a plastic faced hammer, a brass punch or rod, calipers, and a towel/rag.

Begin by cutting your wooden dowel 6″ to 8″ in length.  I’ve used a mini-chop saw and a dremel; however a simple metal saw blade wood work as well – just may take more time to cut.




Next, secure the firearm vertically as you will be needing to use both hands for the next steps.  Place a towel or rag under the butt of the firearm to catch the slug as it is driven through the firearm and falls out of the chamber on to the floor.





To the left are slugs and worm sinkers made of soft lead.  The slugs, on the left, I made myself using a home-made mold.  Absent the slugs, I prefer the worm sinkers as they are shaped like a bullet and center themselves within the muzzle as we work.


This is a mold I made so I can make my own slugs; an alternative to buying lead sinkers.  I used a scrap piece of aluminum and drilled several different sized holes slightly larger than the bores for which I often slug.  These holes do not go all the way through the mold; I left about 1/4″ not drilled through.  Then, on the reverse side (see photo insert) I drilled a hole for each cavity, then tapped the holes.  A screw is then inserted to prevent the lead from flowing through the bottom while it’s being poured.  Once the cavity is filled with the lead and it cools the screw is removed and the slug is tapped out with a punch.

Next we need to lubricate the bore of the rifle with grease thoroughly to ensure the slug rides smooth down the bore. Put a generous amount of grease on a patch and run it down the bore.




Next, place a generous amount of grease on the slug and around the muzzle and place the slug into the muzzle as shown.




Using your plastic mallet, tap the slug into the bore until it is flush with the muzzle.  As you can see, a tight slug will flatten at the mouth of the muzzle.




Using a brass punch, tap the slug a few millimeters more to get it started into the rifling.  It’s at this point you’ll experience the most resistance of the slug as it attempts to engrave the rifling.



Using a length of wooden dowel start tapping the slug down the bore.  As you run out of dowel, add another piece to continue until the slug is driven out all the way through and out of chamber…





…and drops onto the towel.





Now we can measure the lands and grooves of the chamber.  In this tutorial, I slugged an M95 Steyr long rifle. The lands of the bore measured .318″. 




The grooves of the bore measured .333″; which for my cartridge then I will cast lead bullets .334″ to .336″ for a nice, tight fit and gas seal to prevent leading.



Get into the habit of documenting your findings of your slugs.  I store my slugs in a plastic fishing lure container with the respective serial number of the firearm.  I also record the data in a program designed for collecting; namely, NMCollector software.  NMCollector software is a great program if you are serious about collecting and maintaining records of your firearms and accessories.

Taking the time to slug your firearm’s bore will make the difference as to how accurately your firearm will shoot.  It’s an absolute necessity if you shoot cast bullets for firearms that projectiles are difficult, if not impossible, to find.

Safe Shooting!

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