What Cast Rifle Bullets Want
Accuracy may have more to do with the chamber throat than any other single factor, more so with cast than with jacketed. If a gun shoots jacketed bullets tolerably well but hates cast bullets, chances are the throat is at fault. Please measure your rifle’s throat, with a chamber casting or chamber slug, before you order a mold.
My Credentials as an expert on rifles I’ll tell you upfront that I am not much of a rifleman, and only occasionally fool around with cast bullets in rifles. No doubt many of you know much, much, more than I do about this subject. Nonetheless, I get asked for advice on rifle bullets nearly every day. I will try to explain the basic principles involved in getting a rifle to feed and shoot cast bullets, and mention some of the potential problems that beginners should watch out for.
The throat is the part of the chamber between the case and the rifling. In a perfect world, the throat diameter would be 0.001″ – 0.002″ larger than the groove diameter. The throat serves to align the bullet so that it is pointing straight into the barrel. If the throat diameter is much larger than the bullet diameter, then the throat cannot align the bullet. Also, an excessively large throat diameter may make the bullet susceptible to gas cutting as it passes through the throat.
Throat length For our purposes we will say that the effective throat length is the distance between the case mouth and the beginning of the rifling. A long throat is fine providing the diameter is correct. I prefer to have about 0.150″ – 0.250″ throat length so that a groove diameter front band can be chambered. The throat length that is created by an excessively long chamber neck is not a good thing.
Throat angle The bullet wants to encounter a gradual taper between the throat and the bore of the barrel. Benchrest shooters prefer a 1/2 to 1 1/2 degree angle. Many lever action and break open single shot chambers have a much steeper throat angle. These steeper angles may not be ideal for cast bullets. The steep angle does not provide any guidance and may even shave lead as the bullet enters the rifling, whereas a gentle throat angle helps to guide the bullet into the barrel, with minimal damage.
Off-center throats Mass-produced guns may have throats that are not concentric with the barrel, especially if they were chambered with a reamer that had an undersize pilot, or if they were chambered using a drill press. Off-center throats are more common than you might think. I have owned two rifles that had off-center chambers (you could tell just by looking into the breech end of the barrel). One was a 7intR contender barrel, purchased from IHMSA, that I suspect was chambered using a drill press. It shot poorly, even with jacketed bullets. The other was a Magtech .22 LR. The Magtech wouldn’t keep it’s shots on paper at 50 yards, and the bullets were keyholing. I set the barrel back and rechambered it using a portable drill, and now it does 1 1/2″ MOA. Those are two extreme examples, but you have to wonder if there are chambers that are not off-center enough to detect with the naked eye, but enough to hurt accuracy.
Throatless chambers Not all chambers have throats. Instead, the chamber neck ends, and the rifling begins, just like that. Throatless chambers are often found on lever guns and break-open single shots. I; haven’t figured out if throatless chambers are due to a SAAMI requirement or just plain stupidity, but in any event, these chambers are not cast bullet friendly (yes, lever action and single-shot fans, some of these rifles will shoot pretty darned well with select loads, but I’ve heard the other side of the story, too). Without a throat to guide the bullet into the barrel, the bullet hits the barrel off-center, damaging the bullet. Jacketed bullets are tough enough to tolerate this abuse, but cast bullets are not. If you have a throatless chamber, consider renting a throating reamer and giving the poor gun a proper throat (make sure the reamer has a gentle throat angle, correct throat diameter, and correct pilot diameter). If the chamber is throatless yet has an excessively long chamber neck, then consider setting the barrel back 1/2″ and rechambering with a CB-friendly reamer. If you want to try shooting cast in one of these chambers, your best bet is to either have an ogive that snuggles up against the rifling, or a bore riding nose — anything that helps to align the bullet in the barrel despite the missing throat. Be aware that full-diameter ogival bullets may not chamber in a throatless gun. Please measure your throat before you order a mold.
Lever Actions beware of throatless chambers and/or excessively long chamber necks. Many lever guns do not have enough throat length to chamber a 1-diameter ogival bullet and instead require a 2-diameter bullet. Nose length is critical for feeding. Most, but not all, lever guns are pretty good about feeding blunt noses.
Break-Open Single Shots often have throatless chambers similar to lever guns. See Mike Bellm’s articles on TC throats. I have examined one chamber casting from an H&R; 45-70, and it was throatless, though otherwise, all dimensions were quite snug.
Bolt Guns usually have decent throats, but there is a lot of variation in throat length. If your bolt gun has a long throat, it’s a good candidate for a full diameter ogival bullet. If it has a short throat, a bore riding design may be necessary to avoid deep seating. Controlled feed guns, like the Mausers, are usually pretty good at feeding blunt bullets. Push feed guns, like the Remington, may have trouble feeding flat points. When in doubt, make a dummy round with a spitzer bullet and gradually file down the nose to a blunt flat point to see if feeding problems arise.
Ruger #1 I’ve only examined one Ruger #1 chamber casting, a 375 H&H, and it had a generous throat diameter, 0.391″ if I remember correctly. The other dimensions — throat angle, throat length, and the groove diameter — were fine.
Microgroove Rifling I don’t have personal experience with this rifling. But if I did have a microgroove barrel I would be more concerned with the throat than with the rifling.
Seating into the powder space sounds like a bad idea, conjuring images of grease contaminating the powder or grease getting blown out of the grooves or gas checks getting blown off. But I’ve shot thousands of CBs that were seated into the powder space, and it didn’t seem to create any problems. Still, I try to avoid it, just because it sounds bad in theory.
Case Neck Fit Benchrest chambers may only have 0.001″ clearance in the case neck area, but mass-produced rifles typically have 0.010″ or even more clearance in the case neck, with a groove diameter bullet. That means the base of the cast bullet can wobble, causing the bullet to enter the barrel crooked. It means the bullet may be susceptible to gas cutting while it is in the neck. Should we attempt to size the bullet to be a snug fit in the neck? I don’t honestly know, but will try to find out.
Obturation Peak pressures are not reached until the bullet has traveled nearly an inch, meaning obturation, if it happens at all, will not happen until most of the bullet has entered the rifling. By that time whatever bad things that can happen in the throat have already happened. In other words, don’t count on obturation to make the bullet fit the throat.
Sizing the bullet Conventional wisdom says to size the bullet to fit the throat. But many chambers don’t even have throats!!! And what about that Ruger #1 with the 0.391″ throat and 0.375″ groove — do you think a 0.391″ bullet would shoot better than a 0.376″ bullet????? If the gun has a throat, and if the throat is only a few thou larger than the groove diameter, then I’ll go along with sizing the base of the bullet to throat diameter. But if the gun has one of those throatless chambers with the cheese grater throat angle, a fat bullet may get chewed up as it enters the rifling. I honestly don’t know what diameter is best for not-so-great throats. Consider that you can always size a bullet down, but it’s hard to size a bullet up.
Front band diameter There is a danger in making the front band diameter too big. Ideally, we want to be able to seat the bullet out until the front band kisses the rifling. Now if you are lucky enough to have a snug throat diameter, the front band may run aground on the throat before it reaches the rifling. For a sporting rifle, I say make the front band one or two thou smaller than the throat, so it can easily reach the rifling. Case in point: a M700 ’06 has a snug 0.309″ throat, yet it has shot its best groups to date with a 0.313″ bullet (see the paragraph on case neck fit for a possible explanation). Said bullet had a 0.3095″ front band and was running aground in the throat before it reached the rifling. Perhaps a bullet with a 0.308″ front band but fat (0.313″ – 0.319″) driving bands would work better?
Fillers I have never used fillers so I won’t say much about them, except, in addition to holding the powder in place, fillers may also serve to protect the bullet from gas cutting.
Lapping and Firelapping Back when firelapping first became popular, I firelapped most of my guns. It did not improve accuracy, though it seemed to reduce fouling in some barrels. I will not firelap again because firelapping can increase the throat diameter, perhaps as much as 0.002″, and most throats are already too big to begin with. If a barrel is rough, I would suggest the old-fashioned lead lap. On the plus side, firelapping should round off the sharp edges and tooling marks that may be present in some throats, and I suspect that is why some guns have responded favorably to firelapping.
Things that can go seriously wrong
— won’t chamber due to short or non-existent throat.
— won’t feed due to nose shape or length
— poor accuracy because bore riding nose diameter is too small
— won’t chamber because bore riding nose diameter is too large
— bullet can’t be seated to reach rifling because nose diameter is too large (or got bumped up in lube-sizer).