Bullet Seating Sweet Spot

Using a machine rest, I am able to find the bullet seating sweet spot for every gun I build. It takes me about a half day of range time if I have ammo already loaded and nearly a full day if I am forced to load while I proceed. If you don’t have a machine rest or are much more skilled at bench rest shooting than me (almost anybody is) just provide guidance for the owner to perform their own range testing. Talking about "sweet spots" can become very esoteric with terms like nodes, null spots, barrel harmonics, etc.

Even though I have a general understanding of this, these essays can be difficult to follow. Regardless, the goal remains to discover whatever it takes to get groups to be the tightest. Group shrinking is often done by varying bullets, propellants and the amount charged, actually changing the harmonics of the firearm (such as Browning’s Ballistic Optimizing Shooting System), or changing bullet seating depth while observing group Below top: A front bore guide is helpful.

Exercise caution when inserting a cleaning rod down the bore from the muzzle end. The rod is used to push the Stoney Point gauge out the back of the receiver. Below bottom: Loosen the little (about 3/8") brass knurled nut located on the 5/8" x 3/4" sliver collar (roughly to the right side) on the overall length gauge. Pull out the gray plastic insert to allow a bullet to be inserted in the neck of the dummy cartridge casing on the extreme left. sizes to find the a "sweet spot." We’re looking at the last one now.

Combining techniques can help but various restrictions may prevent using them. I’ll also cover long seated ammo with 80 and 90 grain boat tail bullets used for single-fed slow fire events, though the techniques can be adapted for magazine-fed ammunition. Finding the bullet seating sweet spot begins by taking measurements of the cartridge rim to bullet ogive.

You’ll need an overall length gauge (mine is from Hornady), a dummy cartridge casing that screws on the front end of the gauge, comparator that attaches to one of the legs of your caliper, a caliber-specific insert for the comparator, quality calipers measuring to 0.0005", a quality cleaning rod of proper length, ten bullets of the lot intended to load, and an immaculately clean bore and chamber. Exercising caution, insert the cleaning rod down into the bore a short distance from the muzzle end.

It is helpful to have a front bore guide. Insert one of your ten bullets into the neck of the dummy casing attached to the front end of the overall length gauge. There is a collar on the shaft of the overall length gauge with a little knurled brass-colored nut on it. Loosen it to release the gray plastic inner section and pull that back far enough to allow the bullet to drop down in the neck a little further than you expect the loaded round to finish up at.

I use the collar as a grasping point to insert the whole unit up through the receiver and into the chamber. Push everything up until the shoulder on the dummy casing hits against the shoulder of the chamber. Maintain some forward pressure to prevent the dummy casing from backing out. Maintain your hold on the collar with the index finger and thumb, then move the inner plunger forward, pushing the bullet further forward in the chamber.

Stop pushing when you feel the bullet come in contact with the back of the throat. The object is to locate the very back of the throat, not some point 0.002- 0.003" further forward. Without letting up with the forward pressure, lock the knurled brass nut down hard enough to trap the plastic plunger and save the setting. This procedure may be awkward at first but it becomes easy enough. Using a front rod centering guide, keep the cleaning rod traveling straight and carefully push it further into the barrel.

It will eventually come into contact with the bullet on the end of the dummy casing and push the entire unit out the back. Zero the caliper, open the jaws, and place the dummy casing

Above center: A hand press with competition bullet seating die is handy to use on the range to vary cartridge length. This saves trips back and forth to the loading bench between test groups. with the rim flat on the naked leg of the caliper and the bullet "nose cone" extending down into the silver bullet insert that is attached to the comparator clamped to the other leg of the caliper. Close the jaws of the caliper.

I like to give everything a little "squiggle" while doing so to make sure the back of the dummy cartridge is sitting flat on its caliper jaw and that the bullet is straight seated in the bullet insert. Maintain light forward pressure on the thumb wheel of the caliper and record its reading to ten thousandths of an inch (four decimal points). Repeat these steps for each of the remaining bullets and average them. This is the estimated chamber length. Pull the cleaning rod back out part way between each reading.

If it is inserted too far into the bore the bullet will abut against the end of the rod instead of the chamber throat, confusing the readings. Also, loosen the brass nut each time and push each of the ten bullets just into light contact with the back of the throat. Consistency is important. Don’t hesitate to confirm with multiple caliper readings. When moving the overall length gauge from the back of the receiver to the caliper you might accidently dump a bullet out of the dummy casing.

No problem, as your reading was locked in by tightening the knurled thumb nut. I’m often asked how far to seat 80 or 90 grain bullets for best accuracy. No two guns are alike and is the reason each will have a unique bullet seating sweet spot. On top of that, each of us exerts our own unique pressures on the measuring tools and may see slightly different readings. My average sweet spot tests indicate a 0.004" jump with boat tails but I have firearms all the way from 0.000" to 0.010" of initial jump.

Before live fire testing, note that any time we attempt to influence barrel harmonics or optimize ammo it is critical to test with the gun fired exactly the way it is going to be fired in real life. Different bolt carriers or adjusting their weight, or anything touching the barrel (flash suppressor, gas block, "bloop tubes", etc.) can induce a change. After getting the ten bullet average chamber reading, I subtract my average sweet spot jump of 0.004" to get my target length.

I head to the test shed with a tray ammunition with the bullets seated long, a hand press with a competition bullet seating die, and calipers. I adjust the die to final seat my bullets as close as possible to the target length and spin them on a concentricity gauge, marking an index of the high side. I measure the cartridge rim-to-ogive distance of the first ten rounds and record the average. I label that column of readings with the die setting so I can repeat it if need be.

Due to variations in bullet profiles, thicknesses of brass in the neck of the cartridge, and the way propellents may pack in the case, not all rounds come out with the same length reading and may have a variance of 0.002" long or short. Longer ones are closer to the throat. If too long, pressures increase and you’ll see it on the chronograph. A maximum velocity spread of 30 to 40 fps for ten rounds is good. Absence of fliers in the target group is another good indication.

In addition to the gun or upper, each customer gets a sample box of ammo set to the sweet spot. Author keeps a file copy of one round in an envelope for his records. I use a Bob Sled magazine and push every test round into the chamber a uniform depth. My studies reveal this can shrink group sizes by 13.8%. As soon as I finish shooting that first group I go down range and label it along with the die setting used. I return to the shed and look at the length average from the first ten rounds.

If I was short of my target length I reset the die to seat 0.002" longer, or reset the die shorter if the target length was long. The procedure is repeated for the next ten rounds. If the second group is better, the gun is telling me I am going in the right direction and I adjust seating 0.002" more. If not, the gun is telling me to adjust seating depth in the opposite direction.

As long as my groups are shrinking I continue to adjust seating and testing until I find a seating depth where the groups begin getting fatter. Depending on what I am seeing on target, I may try to refine seating depth by going in 0.001" increments around the tentative sweet spot die setting. Once the final sweet spot group is established, I average the ten rounds producing that group to find the sweet spot length.

I load a box of twenty rounds plus one extra and measure each of them to find which ones hit that sweet spot length to within 0.0005". No error allowed in those. I save one of the spot-on rounds and seal it in an envelope with the die setting, date, and customer name or gun number, powder type and weight to use as a template for future loading.

The remaining spot-on rounds are marked and given to the customer to save as a permanent record and template for die settings along with load data used and a copy of the final machine rest test groups. Having done the testing, it’s up to the shooter to duplicate these handloads. No ammo, including rounds seated to a found sweet spot, will perform its best if concentricity is ignored.

For the Army Reserve Team, we issued the exact hand press and die I used when establishing this test work to the shooter receiving the rifle. Besides following good loading procedures to reduce and measure concentricity, the die setting was left in place and paint marks put on as witness marks with load data written on the press handle. Using Team ammo, I even used Loctite on the die body threads so it couldn’t back out and mess up the final seating depth.

We could do this because the Team loaded their ammo with mass-purchased common components and maintained consistency from year to year in neck tension and other factors that can cause differences in average seating depth. Few individual shooters can do this. Tune Ups The sweet spot seating length will last for the life of the barrel. With boat tail bullets the shooter doesn’t have to lengthen the rounds to compensate for throat erosion. In fact, he doesn’t even have to ever take a Stoney Point reading.

However, it is helpful for the shooter to have a tune up from time to time and update his seating sweet spot on an annual basis. The throat erodes forward and bullets jump longer distances. Unless those bullets are lined up perfectly with the axis of the bore they will be deformed and not shoot well. The tune up process starts by selecting ten bullets from the lot used. Measure and average the readings as done previously. Subtract 0.004" to find the expected tune up target length.

Adjust the seating die as done previously. This tune up keeps the gun shooting well at long range until the bullet has been moved so far forward that the neck doesn’t have sufficient grip. As bullets are moved forward significant distances (we’re talking hundredths of an inch here) velocities will drop off. Watch for this on the chronograph. It is not unusual to bump up charges 0.2-0.4 grains while performing tune ups. I’ve found muzzle velocity in .223 Remington changes about 10 fps for every 0.1 grain of powder.

High Power competitors often shoot two different bullets. Most AR-15 shooters use a 77 grain bullet seated to function in a magazine for their 200 and 300 yard stages and hand feed a longer, heavier bullet at

600 yards and beyond which might not fit the magazine. Single fire ammo has been the subject of this article and many shooters judge the performance and longevity of barrels based only on 600 yard performance. We can improve accuracy with long range ammunition as discussed; this isn’t as useful with magazinefed cartridges. Yet, 60-75% of most match aggregates are fired with the shorter cartridges and it’s imperative this ammo performs as well.

The Wylde chamber I often use jumps magazine-length 77s about 0.090" when the chamber is new. As the throat moves forward due to erosion, jump length increases with it and performance can soon drop off. Those shooting only 80 grain bullets at the 600 yard line can extend the total performance with both 77s and 80s by using a chamber with less throat as from Compass Lake or Accuracy Speaks. Those chambers also yield higher end round counts as long as you don’t booger bores with rough cleaning.

My machine rest tests show that the Compass Lake chamber shoots ten shot groups of magazine-length 77 grain bullets 23.3% tighter than the longer Wylde chamber. As throats erode with use be certain to pay attention to concentricity and straighten cartridges as needed. When barrels are near their end it is not unusual for 300 yard accuracy to degrade before a fall off in 600 yard performance is seen.

It’s also worth noting that merely taking anew set of measurements on bullet seat depth after a competitive season and subtracting the jump from the previous year will not automatically establish anew sweet spot. This will get you to within 0.003" but that’s too much error. It’s a good start point but you’ll still need to test with varying lengths in 0.001-0.002" increments to find the updated sweet spot. Bolt gunners in some shooting disciplines sometimes "soft seat" their ammo.

They load their boat tails a little long with modest neck tension and chamber each round singly and very slowly, carefully closing the bolt. This has the bullet stop in the throat while the brass moves up around the back of the bullet as everything is locked up. Some Service Rifle shooters tried this with hand fed ammo seated long and it did not work. Most military shooting teams load their long range ammo hot with new Lake City cases and crimped-in primers.

Soft seating raises pressures and results in blown primers. To make matters worse, the blown primers frequently become lodged in the barrel extension, preventing the bolt from locking up, or under the trigger. This version of soft seating is done by laying the round on the top of a magazine and tripping the bolt catch. Having the bolt to fly forward like that is better called hard seating! Bullets are jammed hard into throat and any need to clear the rifle pulled the bullet, spilling powder into the action.

This also causes groups to open up. Using across section of Team uppers, I machine rest tested ammo seated to the sweet spot compared to "hard seated" ammo to prove this. VLD Bullets Very Low Drag bullets offer some advantages but are high maintenance. Finding their initial sweet spot works like boat tails but where boat tails are jumped some distance, VLDs work best pressed into the throat slightly. To find a VLDs sweet spot, start with the same steps above but begin with about 0.005" of jam into the throat.

Adjust longer or shorter as tested above.

Above and The thing with VLDs is that you don’t just load up a case of them for the whole season and kick back. Remeasure after thoroughly cleaning the gun after each range session. That average measurement gets longer as the throat erodes. Adjust seat depth when loading the next batch or rounds accordingly. This is "chasing the throat" and keeps the same amount of bullet jam as the throat dimensions increase over time. This isn’t difficult but requires attention to detail and meticulous record keeping.

The Army Reserve Team tried some 90 grain VLDs for long range before Berger came out with their very good and user friendly boat tail. While our shooters posted good 1000 yard scores, chasing the throat was a pain in the neck. With boat tails, I find a sweet spot close to the throat. I hear guys talking about finding sweet spots at much longer jumps, sometimes from 0.020-0.040". I’ve tested this but found it wasn’t better then using 0.010" of jump or less.

Not as close as VLDs, but a big jump seldom yields optimum results. As you can tell, I like to quote numbers. I have literally found this bullet seating sweet spot for hundreds of uppers. My records show I’ve shrunk group sizes by at least 20% on average by doing so. Each gun is different but this has always yielded some improvement. The sweet spot seems to allow a powder charge variance of 0.3-0.5 grains and works in a variety of temperatures without causing problems.

It has been many years since I have seen a sweet spot ten shot group go over MOA on my machine rest with most into the 1/2-5/8 MOA range. Plotting sweet spot jump lengths on a graph was a nearly perfect bell curve. Most guns liked 0.004" of jump (measured by my hand) and an equal number preferred 0.005-0.006" jumps as those liking 0.002-0.003" better.

Guns wanting 0.007-0.010" were even more rare than those few wanting only 0.001" Different people will exert different pressures as they push the bullet into contact with the back of the throat for measurement so your average sweet spot jump may be a little different than mine. Keep good records when you test this. Knowing my own average jump as 0.004" saves tons of time.

It is not unusual to fire an initial test group seated at 0.004", then fire two more test groups adjusting in 0.002" increments to see them get fatter and correctly conclude the rifle is yet another preferring my normal average. Magazine-Length Ammo Military shooting teams usually load their long range 80 and 90 grain ammo but buy commercial 77 grain loads for the short lines. The handloads generally shoot tighter groups than even the best commercial 77s. That is a penny above the group.

Since the ammo is already assembled at the factory, "repair options" are quite limited. Fixing run out using a cartridge straightening tool is one very effective method of making factory ammo perform better. I wondered if a version of the custom seating depth methods could work with magazine-fed ammo. Obviously, you can’t lengthen factory magazine-length ammo but you can seat deeper if needed.

Select 10 rounds of ammo from a box of the lot you intend to use, deal with their concentricity, and test fire as a control. Measure the factory cartridge and adjust the die to seat it 0.002" deeper. Seat the bullets, deal with their concentricity, and fire a test group. Continue to seat deeper in 0.002" increments as groups shrink and then start getting fatter.

Unfortunately, this method is very lot specific and changing lots of even identical brand ammo can change the sweet spot depth, though not likely to be more than 0.004". Vary your seating depths back and forth in .001 or .002" increments until you find the new sweet spot. Handloaders using the same lots of components won’t likely need to adjust their sweet spot for years, provided powder and loaded rounds are properly stored.

Most magazinelength ammo will sweet spot with less then 0.010" of adjustment and won’t increase pressure. AG