Such a simple subject however people seem to have many different views on drilling and tapping cases for use with an OAL (Overall length) gauge so as I had some cases to do first thing I took some pictures of the process.
First job is pass a mandrel through the neck to ensure a good sliding fit for the bullet and this is the first potential problem area as the brass spring back can leave you with a case neck that is either under size which means moving to the next mandrel size up. People ask me where I buy my mandrels, the easier answer is I make them to suit the job and have an assortment to suit everything from .17 bullets and upwards with .0001″ steps on some. This image is my ‘every day’ set, these are the mandrels I use most and if they don’t get the job done I have another set to work with.
Next job is pop the case in the lathe, I use the Colchester Chipmaster for this as it is accurate and easy to use and I grip the brass case in a collet chuck. The advantage of the collet chuck is it grips evenly around the circumference of the case and if you look carefully you will see the collet is just sitting over the groove which prevents any pressure marks. In theory I have no need to do this however it is very easy to over tighten a collet chuck and deform the material being gripped and this way works for me.
I drill the primer out with a centre drill however this case came full length sized so I simply drill with a 7,0mm HSS drill, this particular drill is a Dormer Jobber drill that I only use for this job and I have reground it to suit brass.
Now I add a generous chamfer to the hole, this is because burrs can occur during the drilling and tapping process and these lead to incorrect readings when measuring with your OAL gauge so care at this stage ensures you will measure from the case head as opposed to a burr that adds a couple of thou to the measured length. It is always worth running a finger across the back of a drilled and tapped case to ensure there are no burrs evident, if there are you can carefully stone them off however take care not to remove material from the case head itself as it will lead to measurement errors.
Onto the tapping process, I am using an M8x0,75mm Volkel tap. This may sound an odd size to you however it is the thread I use for all of my OAL gauges, there is a long and involved reason for this however the short reason is if it is M8x0,75mm I probably tapped it which takes any doubt away as to the method of tapping. Yes, other people use the same tap however they are usually in the USA or AUS, are using one of our OAL gauges and prefer to not send cases.
I tap at 410 RPM and use a Rocol cutting compound. The tap is held in a drill chuck and although I do use a tapping head for other work it is not necessary for this particular application. (OK, it is convenient to stick with the same chuck for the duration of the job)
We also allow for some cases to be tapped within the price of our OAL gauges.
I do drill and tap to suit the Hornady OAL gauges in 5/16″x36 TPI and I will even carefully polish your brass if requested.
Finally, I am often asked the question, ‘In what state should I send my cases, fired, or fired and neck sized, or full length sized or something else?’
My answer is whatever is best for you. The datum for the tapped case when checking OAL is the shoulder of the case versus the chamber shoulder and you will be pushing the case against the chamber shoulder so if you are setting your shoulders back a thou or so after every firing you will instantly dial in a variance of your 0.0015″ or whatever your setback is. Does this matter, well yes and no, yes in that you get a slightly incorrect measurement. However no as this check will be part of your load development and is a combination of finite measurement and comparison so as long as you always use the same case preparation and reloading process and measure/compare in the same manner you are good to go. Do remember that just because a .012″ jump is perfect for one rifle does not mean it will work as well in another.
I know that is a simplistic answer so maybe I should write something when I have a spare couple of hours 🙂
Right, my coffee is drunk so best get back to work now.