Trim your cases they said, they expand and you need to trim them back they said, well I have an RCBS trimmer. No matter what caliber I trim with it, my groups just about double in size. Today I shot my STS 338 Lapua. Fireformed cases, neck sized, and I trimmed them down to EXACTLY the trim length specified in the Hornady reloading book. Then chamfered and deburred, but did it to where I didn't really take any brass off the neck, just took the burrs off and made it look factory again. I reloaded them the same way I did with the groups I shot yesterday. My groups yesterday averaged 1/4 to 1/3 MOA. Today my groups averaged 3/4 to 1" and the worst part is I felt like I was shooting better because I wasn't tripped out on 4 cups of coffee like yesterday. Does anyone know what is going on? Is this trimmer shit? Does it not trim the case mouth evenly? Is something distorting the neck when I remove the trimmer? I'm never trimming my cases again!
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Groups go to CRAP after trimming brass
- Thread starter ahhshoot
- Start date
Hey, if you don't want to trim your cases ever again, that's your choice. You'll just run into this problem where some cases will no longer fit your chamber or even worse, will be just a tad bit short of that limit and during the firing process will crimp down on the bullet, forcing pressures to spike and potentially blowing your face off.
So I either blow my face off or live with sub par accuracy? Any real advice? Do you have any ideas as to what may be going on or suggestions?
I didn't offer any advice because off the top of my head I can't think of a reason why trimming to spec length would affect accuracy like that. Hopefully someone more knowledgeable will chime in. However, not trimming with a big .338LM can be dangerous so I felt compelled to at least warn you.
I appreciate that. I can't think of anything either, besides maybe it's not trimming the mouth 'flush.' I'm going to try getting jacked up on 4 cups of coffee before going to shoot again and see if my groups tighten back up. Seems just about as likely to be that as the trimming. But honestly, that seems to be the isolated variable and I'm going to load a few of the same lot cases without trimming, and see what happens. I was hoping someone might have had a similar problem and figured out a solution.
Seating depth variation can produce group differences like that, but not trim length differences as far as I know. If the trimmer was somehow affecting neck tension, that could be an issue, but I don't see how that would happen.
You said that you didn't really take any brass off of the neck while chamfering and debuting, just "did it to where I didn't really take any brass off the neck, just took the burrs off and made it look factory again." It is possible that you didn't completely trim off the inner burr, or otherwise left the brass in a state that could damage the bullet upon seating. Put a big fat vld chamfer in the brass and see if that doesn't solve your problem.
You know I might do that. I would like to think I trimmed it back to full factory spec but who knows. I did feel the inside and outsides of the cases and took quite a bit of care when chamfering/deburring, but a lot of that care was not to chamfer too much. I will throw a big scoop of brass out of there on a few and see if that helps anything. What do you mean by vld chamfer? I am shooting berger OTM if that makes any difference.
You can measure your chamber length, then determine whether or not and or when to trim your brass. Take the guess work out man!
Sinclair Chamber Length Gage | Sinclair Intl
Sinclair Chamber Length Gage | Sinclair Intl
Awesome, this makes sense, if my chamber is able to accommodate longer case length than is published in my manual, that would decrease accuracy. One question, would it be close enough to measure a piece of twice fired untrimmed, and unsized brass? Wouldn't that give me a pretty good determination of my chamber length? Thanks btw
Standard chamfering tool...
VLD chamfering tool...
Note the difference in the angle of the cutters. The shallower angle of the vld chamfering tool is supposed to allow the bullets to seat more consistently.
Note how much brass was taken off during chamfering in this picture. Also note that the chamfering on the left is, in fact, concentric but it looks weird due to the angle,of the picture. The angle put in by the chamfer helps get the bullet going into the brass easier and more consistently, especially relative to the very flat trim job of the brass on the right...
VLD chamfering tool...
Note the difference in the angle of the cutters. The shallower angle of the vld chamfering tool is supposed to allow the bullets to seat more consistently.
Note how much brass was taken off during chamfering in this picture. Also note that the chamfering on the left is, in fact, concentric but it looks weird due to the angle,of the picture. The angle put in by the chamfer helps get the bullet going into the brass easier and more consistently, especially relative to the very flat trim job of the brass on the right...
Very nice explanation. So the fact that the thinner brass around the case mouth isn't making square contact with the end of the chamber doesn't decrease accuracy? I always assumed it would be better to leave the brass thick so it sat square. However, if the bullet isn't seated as well or if the case length is too short and none of it touches, I suppose it wouldn't much matter.
Load 15 rounds:
5 rounds of freshly trimmed
5 rounds of trimmed then once fired
5 rounds of untrimmed
Go shoot them all at once and compare group size. This will eliminate most environmental variables. Just make sure your barrel cools between groups.
This way we should be able to isolate the issues.
If they all shoot well, you could have load/shooter issues unrelated to trimming.
If the untrimmed and once fired trimmed shoot well you could have neck sizing/burr issues.
If the untrimmed shoots well and the other two do not then it is probably trimming related.
The point is, this isn't supposed to happen so we need to start troubleshooting to figure out what the issues are. Best way to do that is to start eliminating variables.
5 rounds of freshly trimmed
5 rounds of trimmed then once fired
5 rounds of untrimmed
Go shoot them all at once and compare group size. This will eliminate most environmental variables. Just make sure your barrel cools between groups.
This way we should be able to isolate the issues.
If they all shoot well, you could have load/shooter issues unrelated to trimming.
If the untrimmed and once fired trimmed shoot well you could have neck sizing/burr issues.
If the untrimmed shoots well and the other two do not then it is probably trimming related.
The point is, this isn't supposed to happen so we need to start troubleshooting to figure out what the issues are. Best way to do that is to start eliminating variables.
Trimming cases, in and of itself, does not cause what you're seeing. YOU are doing something to the cases, and I suspect like others that it's your chamfering process. Even a crappy trimmer wouldn't leave a case mouth so unsquare that it wouldn't be sufficiently addressed during the chamfer and deburr process. Start there...
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You will have to cut about half the neck off of a piece of brass, insert the tool into the brass, load the brass into your rifle, then measure the brass with tool in it with a set of dial calipers. More specific directions are in the description on the link I posted.
As far as chamfering, did you chamfer enough so the brass is not catching and deforming the jacket of the bullet when your seating it? You can look at your seated bullet and see if the mouth of the brass has copper jacketing around it.
Is sounds like you've produced inconsistent neck tension in your trimming process...You don't have the motorized RCBS turner do you? Are you trimming after you resize? Although I dislike overworking brass, you may want to run all your brass through the resize die again after trimming, this should restore consistent neck tension.
There can be many causes of the groups opening up, the chamfering is just one and probably a significant cause. Here is an article on case prep, you dont have to do all but if you do the basics. 1,2,3, and 9 it will get yu good enough. I do all except concentricity, which I will start a soon a I can sell a take off barrel and a SF 7.62 brake.
Complete Precision Case Prep within AccurateShooter.com
Complete Precision Case Prep within AccurateShooter.com
Dogtown, if there is space between the case mouth and end of the chamber, then as the case expands under the pressure of ignition wouldn't it cause inconsistencies in pressure curve, rather than fitting snug and creating a more consistent pressure curve as the bullet enters the lands? If you know this will not affect accuracy, then can you explain how?
Here is my process:
-Neck size
-Trim
-Chamfer/Deburr/Clean primer pocket
-Sonic clean 5x (this is why I dont think burrs are the issue, aside from me examining each case and cleaning them out with a qtip).
-Stick in the oven (just 250 degrees for 10 minutes to bake out all the moisture from sonic cleaning)
-Prime
-Charge to 85.00 grains H1000 with Gempro 250
-Seat bullet
I do not have the motorized neck turner. However I did notice that there is a significant amount of pressure required to remove the trimmer from the case mouth. I don't know, so far the only theories that are making any sense whatsoever are the case to actual chamber length variation and the neck tension inconsistencies.
That increased pressure required to remove the pilot from the neck of the brass is caused by the substantial burr created during trimming. How much of the case neck did you have to remove to get back to the "trim to" length? In addition to the problem caused by the burr and not completely removing it by chamfering, the force required to remove the pilot could be inducing excessive runout in your brass.
Sonic cleaning as in using an ultrasonic cleaner? These units are great at cleaning off the carbon and dirt but do not have the polishing ability to remove a large burr from the brass. That is why we use tool steel knives (chamfering tools) to cut the burr off the case mouth...
There is a good bit trimmed off some of them. I need to pull out the calipers and measure what's actually being trimmed. What would be the solution to avoiding a burr during the actual trimming? The pilot is a bitch to remove sometimes.
First I'm new to this, so take it for what it's worth and not sure how it effects accuracy. I reloaded 308 Hornady Amax and followed the Hornady manual to the letter. The case trim length listed was 2.005", but in the SAAMI guidelines this is the minimum case length. The recommended case length was actually 2.015". I also looked in a few other manuals and the recommended case length was also 2.015 to a high of 2.025". So, I can't speak for all the rounds listed, but the Hornady rec was a minimum length.
I was going to suggest the same possibility. It is possible to cause deformities in the case mouth and neck with the trimmer pilot. Not sure if it would really affect groups to a large extent though
Your issues are likely a function of the fact that you really need to actually chamfer the damn cases and you may need to run your necks back through your neck die to recreate consistent neck tension if you are trimming off large amounts.
As has been stated, not trimming your cases is not an option unless you'd like to risk a kaboom from a pressure spike.
As has been stated, not trimming your cases is not an option unless you'd like to risk a kaboom from a pressure spike.
If you are trimming and create a burr like you are describing, your run-out will likely be very high and could affect accuracy. Also, if you have that much of a burr and press a bullet down through it, you are likely skiving the base of the bullet significantly which can also affect accuracy. Then if you take a round that was loaded under the above conditions and fire it, the effects of the burr and the bullet could cause your pressure to be inconsistent further affecting accuracy.
I would recommend learning how to use a deburring tool, a cleaner will not remove burrs from trimming.
You NEED some space between the case mouth and end of chamber precisely because the brass will expand during firing (actually it springs out and then back quickly). If there's not enough room for it to expand, it will crimp down on the bullet instead, keeping it in the case mouth and causing a dangerous pressure spike. Now how much room should you leave? That's debateable and some benchrest shooters go for an absolute minimum (hence the use of the gage mentioned above), but a consistent amount is what's most important and I personally think that has more to do with providing consistent neck tension than anything having to do with the chamber.
Do you have a neck die? If so run your brass through it after you trim, to restore the proper shape and tension. Try groups again. Ain't this fun! Not being funny, but do you have the correct pilot for your trimmer? Mic it to make sure it is not too big or too small. I am sure you will find that properly trimming your brass does not make it less accurate. Something else must be wrong....
Did you introduce the ultra sonic cleaner before the lousy group brass?
The ability of the ultra sonic cleaner to get all the carbon residue from the inside of the necks could lead to
a change in neck tension.
The ability of the ultra sonic cleaner to get all the carbon residue from the inside of the necks could lead to
a change in neck tension.
I have the same trimmer. With your brass secured in the trimmer, move the cutter towards the mouth and as the pilot enters the mouth, push down on the handle and let the pilot center the brass in the holder. On mine, the brass has to raise up just a tad to be centered in the trimmer. Bottoming out the brass does make it bind on the pilot. You can gall the case mouth pretty easy too.
Another check you can preform. Loosen the holder on the trimmer just enough so you can rotate your brass to see if it binds on the pilot. If you hit a spot where it binds, you need to look at what is causing your necks to be off-centered. I found a damaged expansion and decapping rod in my die and replaced it. You can also roll your loaded rounds to see if the bullet wobbles, or buy a concentricity gauge
Another check you can preform. Loosen the holder on the trimmer just enough so you can rotate your brass to see if it binds on the pilot. If you hit a spot where it binds, you need to look at what is causing your necks to be off-centered. I found a damaged expansion and decapping rod in my die and replaced it. You can also roll your loaded rounds to see if the bullet wobbles, or buy a concentricity gauge
Thanks guys it looks like I have a few very good suggestions to try out here. And you can bet I will. That trimmer is very rough on the cases and I noticed last night it does create a burr as its trimming, and it is not entering the case mouth squarely. I don't think burrs during the seating process are an issue, I examined brass I worked with a few days ago and found no burrs on the inside of the case mouth. Could be an issue with deforming the neck/mouth during trimming causing run out or inconsistent neck tension, or im trimming the brass too short. Im going to isolate all of these possibilities and figure out just what's going on. Thanks to all who contributed to this.
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Try this
Anneal, size, trim, neck size, load.
Get a concentricity gauge and check your runout on a non-trimmed case or loaded round compared to a trimmed case loaded round.
Good luck.
Anneal, size, trim, neck size, load.
Get a concentricity gauge and check your runout on a non-trimmed case or loaded round compared to a trimmed case loaded round.
Good luck.
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