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Metallic Reloading 101 - Part 5: Setting & checking projectile seating depth

By Craig Maylam
  •   18th Jul, 2020 Jul 18, 2020, 3:37 PM
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The most frequently asked question by new centrefire cartridge reloaders is how to determine and measure projectile seating depth. At this point we need to discuss why seating depth is important. There are no hard and fast rules but as a generalisation seating depth affects accuracy. However using a standard seating depth will not necessarily condemn your latest handload to inaccuracy.

 

When I’m developing a load I decide on a seating depth (ie; the overall length the cartridge will be) before I fire the first shot. This means that I usually set the projectile jump (how far the projectile moves out of the case before it engages the rifling) to 0.3mm. This is typically done after measuring the throat of the chamber in the rifle I am loading for with the projectile of my choice.

 

Alternatively, I set the overall length of the loaded round to fit within the magazine. After this I run the usual series of loads, taking the data from a manual, and featuring a powder charge increasing steadily from the starting load to the maximum load.

 

After I’ve decided on the best powder charge (by shooting the test loads at targets) I then shorten the cartridges’ overall length by 0.3mm increments to see if I can hit a better sweet spot. The hope here is to wring out that last bit of accuracy. Common sense prevails though, and once the load meets my accuracy expectations I leave it alone!

 

PRESSURE CONSIDERATIONS

Powder manuals usually offer advice on seating depths because they affect chamber pressure. In my .257 Roberts I had the throat of the chamber lengthened by 3mm in an effort to lengthen the maximum overall length of the loaded round.

 

The effect of this was that the base of the projectile ended up in the neck of the case rather than protruding into the shoulder section. This gave me two things: first, the rifle fed much better and second, it lowered the chamber pressure.

 

“Some shooters like to seat their projectiles into the rifling. This means that when they shut the bolt the rifling will engrave the projectile. Some target shooters and bench rest shooters believe this enhances accuracy.”

 

The pressure dropped because by seating the bullet further out of the case I had in effect increased the volume of the case. I was also able to get more velocity from this load without reaching excessive pressure.

 

Some shooters like to seat their projectiles into the rifling. This means that when they shut the bolt the rifling will engrave the projectile. Some target shooters and bench rest shooters believe this enhances accuracy. In the field this can have two effects, first it causes a pressure spike on ignition.

 

This is because there is no space for the projectile to move into before it engages the rifling - it is already touching the rifling before it begins to move. Pressure spikes can be a real problem on very hot days, so avoid leaving your ammo in the sunshine.

 

A 55 grain .223 projectile (left) and a 69 grain on the right. Note how far the 69 grainer intrudes into the powder cavity. If space in the magazine and the chamber allows, the projectile could be seated further out of the case.

 

Secondly, there’s the real chance of having a projectile stick in the rifling when you try to remove the loaded round from the chamber, leaving the projectile behind and filling your magazine with spilled powder!

 

Target shooters almost always fire the shot they chamber so for them seating a projectile into the rifling has no real risks, plus they have cleaning gear on hand. For the hunter however, having a projectile jammed in the rifling with no cleaning rod to poke it out will ruin your day for sure.

 

As a guide, the best way to determine your optimum seating depth to prevent trouble is to go with the factory overall length. These are available from manuals or from SAAMI (Sporting Arms & Ammunition Manufacturers’ Institute).

 

For the .223 the SAAMI dimensions are 54.99mm minimum length and 57.40mm maximum, so you can be sure these dimensions will work.

 

RESTRICTED AND UNRESTRICTED SEATING LENGTH

We need to divide this next section into two basic categories.The first is “restricted seating depth”, and the second is “unrestricted” - although there is always a restriction of some kind.

 

An example of “restricted” seating depth is where you have magazine box length issues or feeding issues. When I am loading ammunition for my Rossi lever-action chambered in .357 Magnum I have to seat the projectiles to a standard overall length.

 

If the round is too short or too long it will not feed correctly through the mechanism and up into the chamber. Generally speaking most magazine-fed handgun calibres fall into this broad category, as do most rounds loaded to function in an AR15.

 

The AR15 is an absolute classic as it was originally designed to fire 55grprojectiles so the 58mm internal length of the magazine was fine. These days the AR, with many longer and heavier projectiles now avalaible for it, would benefit from an extra 5mm length in the mag box... but it’s too late now.

 

The internal mag box or removable clip mag in a rifle also causes restrictions. Unless single loading is an option, then your handload must fit and feed through your magazine.

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Single-shot actions like a Ruger #1 offer unrestricted seating depth provided your rifle has a sensibly dimensioned throat. With any rifle/projectile there will always be combinations that won’t work.

 

An extreme example would be shooting 110 grain round nose bullets in a .300 Win. Mag. rifle that has been throated especially for 220gr projectiles. In this instance, regardless of where the short little 110gr projectiles are seated in the case, they will be a long way from touching the rifling.

 

PROJECTILE MEASUREMENT

Take a set of Vernier calipers and measure the length of some projectiles. There will be minor differences in their lengths, even from projectiles out of the same box. In determining the ideal overall length of a loaded round problems can occur if you used a shorter projectile as a basis for measurement.

  

Should the projectile you measured be exceptionally short then when a longer projectile turns up the load could be dangerous as the longer projectile may bite into the rifling. The opposite could also apply if you had a magic load that needed say exactly 0.3mm jump and you set your seating dies off a long projectile.

 

Craig uses this Hornady comparator set to measure the overall length of a round (see text).

 

How can we best measure projectile length then? The answer is a comparator which measures off the ogive (curve) of the projectile.

 

Hornady produces a competitively priced comparator that has different calibre inserts as shown in the picture, but you can also buy a similar item from Sinclair which is like a nut with a different calibre measured on each flat. Some shooters make their own comparators by drilling tapered holes in the flats of large nuts.

 

WHAT’S THE MINIMUM I CAN SEAT A PROJECTILE IN A CASE?

I like to seat a minimum of ¾ of the calibre width into the neck of a case. If I was to consider .30 calibre (7.62mm) then I would insert a minimum of 5.7mm of projectile into the neck of the case.

 

For .22 calibre the calculation would look like this: 5.6mm x 0.75 = 4.2mm. Generally ¾ calibre is enough to make the loaded round robust enough for general handling.

 

MEASURING THE MAXIMUM LENGTH OF A ROUND FOR YOUR CHAMBER

There are many methods for measuring the depth of your chamber but discussing them all would be an article in itself, so I’ll describe what I think is the easiest. I use the Hornady “Lock n Load” OAL gauge, a very useful tool. Each calibre needs a specially prepared cartridge case with a fine thread in the primer pocket.

 

The case is threaded onto the end of the gauge, then the grey metal centre shaft is pushed into the case below the shoulder and the small lock screw is nipped up. Push the projectile you want to measure into the neck of the case and insert the assembly into the chamber of your rifle.

 

While pushing the outer edge firmly into the chamber, slide the grey inner shaft and the projectile into the throat until the latter contacts the rifling. Tighten the lock stud and then withdraw the entire assembly. The projectile will rarely come out with the gauge, it normally has to be poked out with a cleaning rod.

 

Once the comparator is set up it is used to push the round into the chamber until the projectile engages the rifling.

 

Now re-assemble the projectile into the case with it sitting on the (locked) grey centre shaft and then measure the overall length with calipers as shown in the picture. This will give you the “jam” length, ie; the length of the round with the projectile jammed against the rifling. You will have to deduct a bit to allow some clearance, I would start with 0.5mm.

 

The projectile you used to measure the overall cartridge length should now be seated into a sample case to the jam length you’ve just measured, minus 0.5mm (or whatever you decide) for some clearance.

 

Some rifles have “free bore” or overly long throats. A military 6.5x55 Swedish Mauser will be one of these because they have a very long throat. The best you can do in this instance is to seat the projectiles out as far as you can, ensuring that they still function through the magazine.

 

A good alternative would be to just use the SAAMI standard length. Your sample round should be labelled and kept for reference. This sample round can be also used to check the function, fit and feed of your loads in your rifle.

 

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SEATING DIES

There are many types of seating dies. I personally prefer the sliding chamber type, but most manufacturers’ dies work well and produce reasonably straight ammunition. To set up the die, put in the correct shell holder, insert a sized case, then run it up to the top of the ram stroke.

 

While the case is still at the top of the stroke, screw the seating die in until you can feel the case. Now back the die out ½ a turn and tighten up the lock ring. If you get this adjustment wrong you will bulge the shoulder on the case and the rounds will not chamber.

 

Now you can adjust the seating stem to get the dimension you determined earlier. Start with the round long and quietly move the stem down until you get the length correct.

 

A selection of seating dies, from the left: Vickerman, Redding micrometer adjustable, Hornady sliding chamber (one of Craig’s favourites), and RCBS.

 

Seat another round and confirm your dimensions, then seat all your projectiles. Expect a little bit of variation in seated length and that will be okay as long as the overall length stays smaller than the jam length you measured earlier.

 

On short runs there is no shame checking every round for length if you are concerned. How straight your ammunition is can be checked by rolling the loaded rounds on a sheet of glass and looking at the ends of the projectiles to see if any of them wobble.

 

I have measured the overall jam length of my .270 rounds at 83.5mm, and have decided to seat the projectiles to a maximum overall length of 83.15mm. I had no problems seating the projectiles into the case and the loaded rounds fit and feed okay through the mag box.

 

TESTING TIME

I assembled some .270 rounds with Hornady 145gr ELD-X projectiles and charges of IMR 7977 powder in the following increments, then headed down to the range (see table below).

 

TEST RESULTS
Projectile Charge Weight Velocity  Group Size
Hornady 145gr ELD-X 56.5gn 2715 32mm
Hornady 145gr ELD-X 57.5gn 2865 30mm
Hornady 145gr ELD-X 58.5gn 2950 22mm
Hornady 145gr ELD-X 59.5gn 3030 12mm

 

Notice that groups tighten as the load increases, and there’s a linear velocity gain of roughly 100fps per grain of propellant added. When this relationship drops off to the point where the powder charges are getting heavier and the velocity is not changing, it is not just a chronograph fault - you will have reached or even exceeded the maximum load.

 

This is a dangerous point to be at and you should stop shooting immediately, pull any rounds apart that exceed the load further, and load a lower powder charge weight.

 

A chronograph is a very handy tool for load development - almost indispensable in fact. With your muzzle velocity recorded you can use the various programmes available and make yourself a drop chart to plot bullet drop at longer ranges.

 

With the load and seating depth optimised, Craig shot the group shown. Now he’s ready to zero the rig and go hunting!

 

The clear winner in the .270 competition was the charge of 59.5 grains - the 12mm 3-shot group met my expectations based on what this rifle has achieved in the past (if I had read the wind better the group would have been even tighter).

 

I used this data to load 40 rounds then went back to the range and sighted the rifle in, producing a drop chart right out to 400 yards.

 

Safe reloading.

 

Craig

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