Size really does matter...Who wants something that's too small or big? Mostly people want to make sure things fit right, comfortably, and can operate as intended and desired. Come on, you know I'm right.
This Wednesday, I've been in my workshop messing about with some gun things because in the not too distant future shooting will resume after shutdowns due to the flu. So today's #whereonwednesday post comes to you from my workshop.
This post will cover a few concepts around size when applied to making ammunition for long range shooting purposes. As you may know, I make my own and this is one of the steps required to be able to make precise hand-loaded ammunition; It's the process of determining the overall length of a completed rifle round. I will refer to it as OAL from this point forward.
I will, as always, not get too technical and will try to explain the concepts and equipment in a way that is understandable for people that may not shoot guns, or reload ammunition. So, here goes, time to load and send it!
Overall length - and why it's important
Firstly, a rifle round needs to fit into the magazine, quite obviously. It also needs to be the right length in the chamber so that the bolt can close and it actually fires. It could be a little short, but too long and the projectile may press too hard on the lands of the rifling in the barrel and cause complications like excessive back pressure. Lands are the raised sections of the rifling in the barrel - The rifling provides spin to the projectile for stability in-flight.
Factory rounds are made to predetermined OAL's and always fit the intended rifle but when making my own ammunition I have the ability to adjust the length by seating the bullet further in, or out, of the case neck. This also allows room in the case for additional amounts of gun powder which may be required or desired. (A whole different post there.)
Seating the projectile in a more shallow position can also have the benefit of closing the gap between the projectile and the rifling of the barrel as the round sits inside the chamber. This gap is called the jump, essentially the gap the projectile has to travel once fired before it engages the lands and heads on down the barrel.
Some gun/projectile combinations like a different jump; Some are fussy and some are a little more easy-going. The rifle you see here isn't too fussy, but my .308 was very fussy and shot best with only a 20 thou jump, so 0.02 of an inch. A very small measurement.
So, how do I determine such a precise measurement to allow me to then manufacture those rounds to a set specification? With an OAL gauge of course.
You can see one below, the long red thing, along with the other bits and pieces required to do the job.
Above you are looking at (from the top):
- 6.5mm creedmoor long range gun
- Vernier caliper
- Comparator gauge
- Projectile
- Modified case
- OAL gauge
These are the bits required to gauge an OAL, along with a cleaning rod (too long to put in the photo), pen and paper.
The modified case - This is a case that get's screwed onto the end of the OAL gauge and inserted into the chamber. The picture below right shows the base of it. It is modified meaning it has a hole in it that fits the gauge. These are calibre-specific. This one is 6.5mm creedmoor and I have a .308, .243 and 7mm case also, all used in the respective rifles.
The process
The modified case is screwed securely to the threaded end of the OAL gauge and a projectile is inserted into the neck as per the image above left; The projectile is free to move in and out. You will note the grey rod that runs through the gauge...This allows the operator to gently push the projectile forward. Of course, this is only done when the OAL gauge is inside the chamber.
The gauge gets inserted into the chamber and the modified case sits there exactly where the actual rifle round would sit to be fired. Below you can see the gauge inserted into my long range gun on the left and the right shows the case on its way into the chamber...It's dark in there so I held it out a little for this image. Note the gap and flat section on the OAL gauge, below right...We will come back to that in a little bit ok?
So this is where it get's a little tricky.
I simply push the grey plastic inner shaft until the projectile stops; That's when it touches the lands of the rifling as mentioned above. At that point I tighten the locking nut, seen in the image below left, and that sets (locks) the gauge.
This has to be done with a deft hand and the same every time. Press even a little too hard and the measurement will be false. Remember, we're dealing with thousandths of an inch measurements here. I use my trigger finger for this part and apply the same amount of pressure I would for a trigger pull, about 2.0-2.2 lbs of finger pressure is where I set my triggers, so it's a fairly light press.
Once the lock nut is set the OAL gauge is taken out. Keep in mind here that the projectile is still stuck in the lands of the barrel; That's where the cleaning rod comes into play. I simply insert it from the muzzle-end of the barrel and tap the projectile out - It doesn't take any real effort.
Now I have the OAL gauge out of the gun, and set, it's time to measure.
The setting of the gauge simply locks the grey shaft into place so it won't move. It's important to make sure the flat section of the shaft is face up in the gap in the gauge as the image above right as that will permit access for the vernier caliper.
I simply drop the projectile back into the case neck and it will sit exactly where it did when in the chamber of the gun for measurement which you can see happening below.
The vernier caliper is zeroed and set to inches or millimetres (I use inches) and the measurement is taken.
I repeat this entire process ten times then remove the highest and lowest measurements from the list and average out the other eight to gain the overall length (OAL) measurement I will work from.
Below you can see the actual measurement taking place. Note on the right you can see I have seated the vernier caliper on that flat section of the inner shaft and snug up against the base of the modified case allowing for a very accurate reading.
As I said earlier, my .308 rifle was very picky about the jump in testing and was far more accurate with a minimal 20 thou jump so I had to set my OAL measurement with that in mind.
After the ten measurements and deleting off the top and bottom ones I ended up with an OAL of 2.3296 inches. Subtracting off the 20 thou as above, to keep the projectile off the lands, I was left with 2.3096 inches as my final OAL.
That measurement ended up becoming the length at which I make the rounds and some precise work is required to set my press to that measurement. This is done manually, there is no gauge for that. It is a long process and one I have to repeat each time I reload for that calibre. I always check and re-check as precision rounds are what I am after.
So that's about it although...
I haven't yet mentioned the comparator gauge so here's a little word on that.
Because the tip of a projectile can vary its an inaccurate way of gauging measurements from. So, we use the ogive of the projectile instead. The comparator gauge allows this to happen. They are simply a gauge that screws onto the vernier caliper and allows the projectile to seat inside it at the ogive of the projectile which is constant and precise. Consistent measurements are now possible. Of course each calibre will have its own comparator gauge and again, I have many different ones.
Below you can see a couple of close ups of the 6.5mm creedmoor comparator gauge used for this post. The hole in the gauge and the right shows the projectile inserted; Where the projectile meets the gauge is the ogive.
I threw this image below into this post because I like it...There's really no relevance to the post other than the fact it is a 6.5mm creedmoor projectile, the Sierra 142gr. HPBT (Hollow point boat tail) and in the background is the comparator gauge.
So to wrap it all up...This is not the only way to determine overall length although it's my preferred way. My friends do it differently on occasions and sometimes I wear a bit of flak for using this gauge but it's all good because the guy that sends the flak is a shit shooter! Lol. For me it's simple and effective method and that sort of sums me up as an individual too, so it works.
Again, I have tried to keep this post a little basic but get the message across and I'm sure if some gun-dude, or dudette, reads it there may be differing opinions and that's ok...I've never professed to be a know-it-all gun guy, just a dude who knows some stuff about some stuff and that's good enough I guess.
Thanks for reading.
Design and create your ideal life, don't live it by default - Tomorrow isn't promised.
Be well
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