Last month we discussed seating depth and the concentricity of reloads; this month we’ll look at different ways of measuring seating depth and how to determine the amount of clearance for each different rifle and loading.
A good starting point is somewhere between .030 to .060-inch.
Several methods have been recommended for determining the particular throat length in a rifle chamber. They all work, some well and some not so well, and some are over-complicated, while others are none too accurate.
One method that often gets a mention is: seating the bullet shallowly- just deep enough in the case to ensure it stays put, and then chamber the round and close the bolt. Extract the dummy cartridge, measure the length of the land marks on the bullet and turn the seating stem down that much plus an additional one thread. In theory it sounds logical, but in practice it doesn’t work for a couple of reasons. First with a long-necked cartridge you may not be able to apply enough force to get the bolt to close because of the amount of pressure required to seat the bullet deeper in the case. Second, if you did manage to get the bolt to close fully, more likely than not when you extract the round, the bullet would be left behind, jammed in the rifling. Third, in the unlikely event you do manage to get the bolt to close, and are able to extract the round intact, the true length of the land marks is just about impossible to measure, and the shape of the bullet at the point of contact is likely to give a false reading.
A more reliable method is to size the neck of an empty case about 1/16th inch and seat the bullet out far enough to contact the lands, then use a candle to smoke the ogive of the seated bullet until it is uniformly coated with black soot. When the soot is dry, chamber the dummy round in the rifle, close the bolt, then open it and carefully extract the round.
Examine the bullet closely with a magnifying glass. If the bullet made contact with the lands when chambered, the rifling marks will show up; if the sooty surface is unmarked, it didn’t, and you can be sure the seating depth will set the bullet off the lands.
If you can see the land marks on the smoked bullet, turn the seating stem down one half the amount the bolt lacked in closing, resmoke the bullet and try again. If it proves difficult to close the bolt, don’t force it, repeat the operation and try again. When the bolt will finally close with little or no resistance, there should be land marks on the smoked bullet. Turn the seating stem down about one half turn and check again to see if there are any land marks. If the bullet just touches the lands, turn the stem down another 1/32 to 1/16 inch and you have the correct seating depth for an accurate troublefree load with that bullet.
Be warned! There’s a problem that might arise during extraction in rifles equipped with a plunger-type ejector mounted in the bolt face. These ejectors will tilt the dummy round to the side as it is withdrawn from the chamber and may cause a streak of soot to be scraped off the bullet in an irregular pattern, but the land marks should still be plainly visible all around the circumference of the bullet.
Each different bullet to be used in a given rifle, even those with apparently similar profiles will require a different seating depth. Round-nosed bullets for example normally have to be seated reslatively deeper than spitzers of the same weight because their fatter ogives will contact the lands sooner as the cartridge is chambered. But homogeneous copper or gilding metal spitzers such as the Barnes X, Hornady GMX And Winchester 95/5 which are quite a bit longer than conventional cup and core bullets will need to be seated deeper. Unless you are content to limit yourself to one particular make and weight of bullet, you’ll be constantly changing the setting of your bullet seating die.
The easiest way to measure the length of a rifle’s throat is to simply run a case into a sizing die just far enough so that the first 1/16th inch or so of the neck is sized down. Take a short bullet that has a flat base (the sharper the base the better), and start seating the bullet in the partially sized neck nose down and base up. If the bullet is short enough and the throat long enough, you may even be able to start it with your fingers, but it is better to seat it down a little with a seating die. Feed the dummy round into the rifle chamber and try to close the bolt. If it proves very hard to close, you will have to extract it and seat the bullet a bit deeper so that bolt will go far enough forward for its camming action to come into play. When the bolt can be fully closed on the dummy round, the bullet will be seated so that its base protrudes the exact length of the throat.
Don’t waste time using a boattail bullet; a bullet with a flat base will not be forced into the lands to give a false reading - and possibly stick. But while this exercise will give you the throat length of a particular chamber, and a measurement to start from, it will not tell you how deep to seat different bullets in that chamber for the correct amount of bullet-to-land clearance.
Another way is to make up a dummy round with the bullet seated to standard factory length. Chamber this round in the rifle and then run a cleaning rod into the muzzle until it makes contact with the tip of the bullet. Mark the rod at the muzzle. Then extract the dummy round, point the muzzle down and drop a loose bullet point-first into the chamber. With the cleaning rod, tap very lightly once or twice on the base of the bullet to make sure it makes firm contact with the rifling. Taking great care not to dislodge the bullet, lay the rifle on its side on a bench and reinsert the rod, allowing it to rest very lightly on the bullet’s nose. The rod is again marked at the muzzle and the distance between the two marks will be the difference between the overall length of your dummy round and the longest feasible seating depth of that bullet in that rifle. This method may sound rather complicated, but it’s not; it’s fairly easy and extremely accurate.
When we are talking about seating depth we are referring to the amount of “jump” the bullet makes before contacting the rifling. Each barrel/cartridge combo has a certain amount of bullet jump that provides the best accuracy. Usually a rifle prefers the same amount of bullet jump regardless of what bullet is being shot. As we have discussed finding the bullet jump that produces the best accuracy is largely a matter of trial and error. Sinclair makes a Seating Depth Tool that makes it easy to obtain the correct overall length, and there are other brands.
I have given up worrying about overall cartridge length in favour of making more accurate measurement from the head of the case to a point on the ogive of the bullet. This is done by using a couple of gadgets - the Sinclair Seating Depth Tool and the Sinclair Bullet Comparator. This handy little gauge is made of high grade stainless steel and has six precision-machined flats each having a cavity for a different calibre that are drilled and reamed with the proper calibre throating reamer. This duplicates the throat of your rifle so that when you measure a loaded round or bullet, it is as close as you can come to duplicating your barrel. The one in the photo is for bullets of .224, .243, .257, .264, .284 and .308 calibres, but I have another for larger calibres.
After using the seating depth tool to determine the optimum bullet jump, I use the comparator together with a caliper to measure from the head of the case to a point on the ogive of the bullet in a dummy round that contacts the rifling.
The Sinclair Comparator will measure further down the bullet than any other tool I’ve used. But it is important to use a light touch with your caliper to avoid crushing the bullet in the gauge. When I’ve obtained an overall length for a particular bullet I use the Comparator to assemble a dummy round, which is used to adjust the bullet seating stem.
Incidently, the Comparator can be used to check the rifle’s throat erosion. You can measure a new round that just touches the rifling and compare it with your original dummy round and then adjust your seater so your reloads are a little longer, to gain the same amount of bullet jump you had before.
Crimping is another operation that is critical in bullet seating. Some bullets are cannelured, including some intended for use in lever-action rifles with tubular magazines and some big dangerous-game calibres so they will not be driven back into the case by heavy recoil. I am often asked how to crimp a bullet that does not have a crimping cannelure, or how to go about it if the cannelure is in the wrong place for the seating depth required.
The type of crimp used by most handloaders relies upon the presence of a cannelure. This is called a roll crimp, and most standard bullet-seating dies come with a shoulder in the die cavity which, when the mouth of the case is forced against it, actually rolls the case lip inward into the cannelure, to lock the bullet in place. Without a cannelure, there’s no place for the lip to go and the case shoulder is usually buckled or bulged and the case wrecked.
This can also happen if the die is set down against the neck mouth of a new case, and it later lengthens after repeatedly being reloaded. Crimping can give rise to problems. If the crimp is excessively heavy or if the bullet does not have a well-formed cannelure, or if case lengths are not uniform, the case shoulder may buckle as pressure is applied, and become misshapen enough to make chambering difficult or even impossible.
All cases to be crimped should be first trimmed after sizing to be exactly the same length in order to ensure a uniform crimp. Afterwhich case mouths must be deburred and chamfered to facilitate the starting of the bullet. Most large calibre bullets intended for large calibre big-game cartridges have the cannelure in the proper location, so that seating depth automatically determines overall cartridge length.
If a cannelure is mislocated for your purposes, simple canneluring tools are available from Lee and C-H which will allow you to put a cannelure on any bullet wherever you want it. I prefer not to crimp rifle ammunition at all unless I have to. Constant crimping and resizing as a case is loaded repeatedly cold-works the brass at the case mouth and eventually results in frayed neck mouths. In most cases, crimping is a waste of time when reloading for medium-calibre bolt-action rifles, but some bullets for them do have cannelures and may even have some two cannleures for use in two different cartridges, others, especially target and varmint pojectiles, are uncannelured.
Some modern cartridges with extremely short necks simply do not have enough bearing surface to to grip long, heavy bullets securely enough to to hold them against heavy recoil without a hard crimp. If unfired cartridges removed from the magazine after the rifle has been fired have bullets pushed deeper into the case than when seated them, crimping may provide a temporary solution but it’s better to locate the problem in your sizing die and correct it.
Most dies allow bullet seating and crimping to be carried out simultaneously. As a rule this works all right, except that the geometry of the dies is such that crimping begins before the bullet is fully seated. With a wide cannelure and careful adjustment of the die it may still work. However, for the best precision and best accuracy, seating and crimping are better done in two separate steps, even though the same die is used for both purposes. In the first step the die body is backed off a turn or two, the bullet stem adjusted to the correct seating depth and the bullets are seated. Then, the seating stem is turned out so that it no longer contacts the bullet, and the die body is screwed in to produce the desired degree of crimp, after which that batch of reloads is run through the die again.
Once the crimping shoulder makes contact with the mouth of an empty case in the shellholder with the ram fully raised, extremely delicate adjustments can be made ; it doesn’t require much screwing-in of the die body to produce a heavy crimp. However, a light to moderate crimp is generally enough to serve its intended purpose and avoid buckling of case necks and reloads which chamber with difficulty or not at all. Care should be taken to ensure that the case mouth aligns properly with the centre of the bullet’s cannelure.
Crimping is largely dependent upon paying careful attention to resized case lengths, and may even require trimming after every resizing. Bullets with very shallow cannelures which are barely knurled make it very difficult to achieve a decent crimp.
The new Lithgow Arms Crossover 101 Rimfire is tantalisingly close.