Measuring Your Bore

Measuring your gun bore for optimum cast bullet fit and performance



Optimum bullet performance, cast or jacketed, is impossible if the gun's guiding surfaces aren’t capable of holding and starting the bullet in perfect balance. The causes of inferior accuracy and excessive fouling, with lead especially, are: rough internal finishes, eccentric chambers/throats, oversize throats and, or, bores, rapid changes (roughness) in twist rate, and constrictions or lumpy bore diameters.

Constrictions or diameter changes are almost certain where revolver barrels thread into frames. Although manufacturers have corrected this considerably since LBT made the public aware of the problem. All guns can have bore diameter changes at any point along the barrel where it was machined, stamped, or welded on the exterior, including contouring, etc.

The worst possible condition is probably a bell-mouthed bore, or enlargement at the muzzle, and, unfortunately, it’s not uncommon. For your personal information, jacketed bullet fouling only causes inaccuracy when it builds up enough down close to the throat that the muzzle becomes bell-mouthed. This will make it more bell-mouthed and less accurate with jacketed. With cast, the increasing bore diameter as the bullet travels down the bore, causes the rifling to lose its grip on the bullet, and worst of all releases hydraulic lube pressure which is essential for obtaining high velocities. Lapping cures the problem.

If you haven’t read that elsewhere, it is because people have not been able to feel out and measure bores. Push through slugs measure it exactly, and are how I learned it!

The following list of items will help you turn most production class barrels into premium performers, and in fact improve most premium barrels, for use with jacketed or lead. Our push through slugs will let you feel the problems and our lap kit will fix them.

The rifle throat slugs are primarily for fitting cast bullets, and will find rifle throat sizes EXACTLY, and show whether they are out of concentric or off center with the bore and all other bad throat conditions.

With the bore lapping instructions provided in the lap kit,  we show you how to make the best of the worst and get good performance. To put it in a nutshell: shooters commonly believe that if a rifle will not shoot jacketed accurately it will not shoot lead accurately either.

However, if a rifle won’t shoot jacketed well because of internal problems, our goodies will tell you why, no matter what the cause, and normally can make it a stellar performer with lead or jacketed.


Rifle Throat Slugs - Package of 2

Price $4.00 Ship Weight 1/4 pound

These are a smooth cylinder of soft lead for taking an EXACT impression of rifle type throats. LBT developed Rifle Throat Slugs because no chamber cast is accurate enough to be useful for cast bullet fitting.

First you need to obtain all of the items you need to slug your barrel.

  1. A Rifle Throat Slug Kit from LBT.
  2.  A 3 foot section of cold rolled steel rod which fits your bore as closely as possible. This is available from most hardware stores.
  3. A fired case of your rifle's caliber, filled to the top of the case with lead.
  4. A large hammer

Insert the lead filled case into the chamber and close the action. Rest the butt of the rifle on the floor and insert the Rifle Throat Slug into the muzzle end of the barrel. You may need to use a piece of wood or similar hard non-marring material to start the slug. Carefully set the steel rod against the end of the slug and tap lightly with the large hammer. Your intent is to swage the slug over the end of the lead-filled rifle case and fill the throat portion of the bore.

One should use Rifle Throat Slugs as a reference when loading jacketed, but we consider them mandatory for fitting cast, to determine the most suitable bullet to fit the throat. The round cold rolled steel rod is softer than the barrel, so it will not damage your bore in any way if the instructions which come with the slugs are followed. These slugs are available for all calibers. Be sure to specify your caliber when ordering. Only one per gun is needed, if done right. We send two in case of a goof up.

To select one of our straight bullets, measure your throat slug to find the full diameter bearing length which will fill the cartridge neck, yet contain the bullet base or gas check, with the forward driving band in contact with the throat taper or rifling. Our spitzer chart shows a bearing length, and the LFN’s overall length and ogive length, which, when deducted, gives the bearing length.

If your rifle has a very large throat and chamber, send the throat slug with the cartridge it was slugged on with your order. Also send a dummy round loaded with a jacketed bullet so I can determine how much larger the cast bullet can be over the jacketed sample. I'll cut a mold with a body that fills the case neck and start of the throat with a large diameter, and reduce the nose to whatever groove diameter is at the point where rifling begins. Sizing in a standard sizing die will provide a close chamber fit and a quick grab on the throat as the bullet starts forward, while the groove diameter nose will be engaged and centered in the rifling before takeoff. I know of no better way to build a bullet for large throats and chambers than this.

Keep in mind that our bullet designs are very strong, and will shoot superbly without all the traditional fitting and fooling. After all, we have many customers who can shoot our revolver bullets into 1 inch groups at 100 yards with revolvers, which says something for their design strength! Rifles are far more gentle on bullets than revolvers, so you can expect our designs to give accuracy without a bunch of traditional cast bullet voodoo.


Revolver Measuring

Because it is critical that revolvers be measured accurately before and during lapping AND when selecting a bullet mold diameter we developed push through slugs, which are listed and explained a bit farther along. For example, if one attempts to lap a revolver barrel while the cylinder throats are undersized, the effort is wasted. The same thing applies if bullets are fitted to undersized throats, or fitted undersized to oversize throats. Don’t guess about bullet fit or a gun's internal dimensions. It’s as erroneous as not looking at the sights when trying to print tight groups.

To measure for a mold using push through slugs, only three are needed in most cases, and detailed instructions come with the push through slugs. Barrel to cylinder throat relationship can be proven by feel as explained in the slugging instructions sent with the slugs.

It is crucial to accuracy with revolvers that barrel constrictions from barrel to frame threads and roll stamping on the outside of the barrels be removed, and any roughness smoothed up, which can ONLY be done by lapping. Push through slugs will make the necessary internal measuring easy, and our lap kit will fix the barrel and cylinder problems. Please read the following information about each carefully.

For optimum results with any rifled firearm we consider it mandatory to know the critical interior dimensions which guide the bullet. By knowing the shape and size of a gun's throat and bore, the jacketed bullet shooter can more wisely select an appropriate bullet, while the cast bullet shooter can both select the most suitable bullet design and fit it to the gun properly.

Push through slugs are one of LBT’s latest developments, which make it possible for anyone to manually feel how the barrel grips the bullet as it travels down the bore, and especially lets you feel whether it has a firm grip as it exits the muzzle, giving a very precise indicator of the barrel's accuracy potential. (The first time you feel the jerks and bumps your bullets feel you WILL be shocked! Everyone is!) They are used for obtaining a precision impression of cylinder throat diameters of revolvers, and groove diameter at various points inside rifled barrels. Measuring the slug with outside measuring equipment such as a micrometer, dial caliper or electronic caliper will then give an exact diameter of the hole through which the slug was pushed.

We recommend push through slugs for checking lapping progress, both by feeling out the barrel and by measuring.

Push through slugs have a tail that slip fits into standard cleaning rods to hold them firmly while reaching through actions, throats and chambers, etc. LBT offers push through slugs for all calibers from 22 to .50. Be sure to specify whether it's for rifle, revolver, or auto pistol when ordering, as several calibers are different in rifles than handguns. We can’t send the correct diameter if we don’t know what gun it is for.


LBT Bore Lap Kit

Price $18.50 Ship Weight 1/4 pound

The current LBT bore lap formula is a highly concentrated abrasive compound developed and manufactured by LBT specifically for lapping barrels by firing lead bullets which have been impregnated with it. (Which is why some call the process fire lapping.)

It cuts aggressively yet smoothly, without excessive throat wear. It is field proven on thousands of barrels, with no bad reports, while most get from good to dramatic accuracy improvement and reduction of bore fouling, using either jacketed or lead bullets. It improves bores by straightening bore diameter end to end, and by smoothing out rough finishes and variations in twist rate, which is accomplished primarily by straightening and smoothing the leading edges of the rifling lands. There are few things a shooter can do as rewarding as running a push through slug through a lapped bore that felt cobble stones before lapping, and feel the slug slide through tight all the way as smooth as glass. Then you see the improved groups on target, knowing you fixed it yourself.

The lap kit contains very detailed and complete instructions and enough abrasive compound to lap anywhere from 100 to 500 guns, depending on gun type, bore condition, and how liberally it is used. The lap compound is used with customer supplied cast bullets. (DO NOT USE WITH JACKETED, AND BY ALL MEANS DO NOT PURCHASE ANY LAP KIT OR SUPPLIES WHICH USE JACKETED BULLETS. DOING SO WILL ABRADE THE THROAT EXCESSIVELY AND WILL NOT DO A PROPER JOB OF LAPPING A BARREL.) The abrasive compound doesn’t deteriorate with age. This compound and instructions are all you need to lap 22 rimfires, including 22 magnum, by applying it to low velocity 22 ammo.

Notice that our kit has one abrasive grit only, and is intended ONLY for use with lead bullets. Any lap kit using multiple grits and especially jacketed bullets will shorten barrel life dramatically and not do a proper job.


Lap Rolling Plates

Price $10.00 Ship Weight 1/2 pound

Precision ground cold rolled steel plates used to impregnate bullets with lap compound to prepare for bore lapping. You can use glass or make your own whatever size you like. We use a 1/8 x 1 x 4 bottom plate and a 1/8 x ¾ x 6 top plate, which is an ideal size to handle and work with. If you make some, we recommend you try this size. Lap kit instructions tell how to use them.


Lead Bullet Technology

78592 Hwy 2
Moyie Springs, ID 83845

(208) 267 3588


© Copyright 2023, all rights reserved.