Hello,
I am in the market for a new trimmer that can chamfer and deburr as well, but I need help with advice. Speed is not important for me as I do a very low volume, accuracy and consistency are very dear to me, and my rifle
First I don't understand the pros and cons between trimmers that trim based on total case length and the ones that index on case shoulder? Please educate me.
I currently have a Lyman universal trimmer, it is fairly accurate, however, I don't like it because it leaves marks on the inside of the neck. My smith manufactured a cross over for an RCBS 3 way trimmer head, same results leave marks on the inside of the neck. It's not the trimmer wabbling, I check it with a XXI concentricity gauge.
Let me know what your thoughts and advice are ...
TIA
You ask a good question regarding using the cartridge base as the datum for measurement vs using the shoulder. In most chambers the shoulder is what sets the cartridge position relative to the throat. This discussion would exclude belted cartridges as they headspace off the belt, but for most other cartridges it makes little difference how you trim UNLESS you tend to trim your brass with the purpose of reducing carbon buildup at the neck/throat transition. In other words, trim to fill the chamber.
If you own a borescope, or borescope camera you are familiar with this area because we are frequently cleaning out the carbon build from the chamber and throat.
If you trim by using the base as the datum, any changes in cartridge length as measured from base to shoulder datum will cause like changes in neck length, and therefore changes in how much of the chamber neck is occupied by the cartridge brass, and how much is open and subject to carbon buildup. As you can imagine, this also causes changes in actual cartridge neck length, and to some degree changes in neck tension as well. Neck tension is a primary factor in load consistency when shooting for small groups.
The problem with carbon buildup is that after shooting a series of loads where the cases are well short of the end of the chamber and has developed a significant carbon buildup, and then a longer case is loaded and fired, that case may not have sufficient chamber clearance to release the bullet properly and potentially cause a pressure spike.
By using the shoulder as a datum, you are using the same datum point the barrels chamber uses and you'll keep a reliable distance to end of the chamber, and that makes it easier to control carbon buildup.
For those very experienced hand loaders who control shoulder bump very precisely, it matters not. For those who are newer to the sport and less able to control shoulder bump while sizing, or aren't sure how to size properly, failure to keep the cartridge mouth out of the chamber throat can lead to a damaged rifle or worse. I've seen this first hand, and it's not funny when gun parts go flying.
I prefer trimmers such as The World's Finest Trimmer and The World's Cheapest Trimmer that use the shoulder as the datum. I typically chuck one in my bench mounted drill press and can trim at a rate of 10 to 15 per minute with very good accuracy.
In the final analysis, having and using a borescope camera will best answer your questions. I frequently chamber a ready to load brass and look at it's fit to the chamber using a borescope camera, especially when asked to check out loading issues for a fellow shooter. I can see exactly where the cartridge mouth is relative to the chamber/throat transition and be sure to trim .005 to .010 off that dimension.
https://www.littlecrowgunworks.com/
http://www.newhighpower.com/brass-trimmers.html
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