case length ?

[rem 243]

Hi everyone,

Have a question regarding case length and trimming, I have full length resized 300 rem saum down to 7mm.

The max case length is 2.035.

I have resized 50 cases and they very from 2.014 down to the shortest at 2.008.

First question is are these too short ?

Second question is should I trim all cases to a uniform length ie 2.008 or should I just load it and let them grow and trim when they get close to max length ?

Bear in mind I am chasing as much accuracy as possible.

cheers and thanks in advance Steff.

 

[fdshuster]

First of all: Determine the actual length of your chamber. They all vary depending on the chambering reamer used. Whoever cut your chamber may have a reamer print drawing, and they could tell you the chamber length.

Or, for a cost of about $6, purchase one of the Sinclair chamber length gauges, page 30 in the Sinclair 2014-A catalog, item# 749-000-784ST.

You can also make a "gauge" as shown in the attached pics.

When you know your chamber length, you can let the cases stretch to whatever length you are comfortable with, and keep them all trimmed to that length. For the cartridge you described I would want a minimum of .010" clearance.

Some cases, by their very design will stretch more than others, and I would expect yours to lengthen after only a few firings. Just be sure to measure the lengths after sizing.

 

[dturnersr]

First of all: Determine the actual length of your chamber. They all vary depending on the chambering reamer used.

When you know your chamber length, you can let the cases stretch to whatever length you are comfortable with, and keep them all trimmed to that length. For the cartridge you described I would want a minimum of .010" clearance.

Thanks for posting this home made "tool." I also have been pondering the effect of using 300 Rem SAUM cases that have been necked down to 7MM 2.005 versus Remington or Norma 7MM SAUM 2.025 cases that of course have much longer .20 necks. I measured my R700 chamber at just 2.065 twice once using a true 7MM case and also a .300 case that had been necked down. So noting the true chamber cut size I will let some of my cases "grow" and reverify the length as I have not been trimming the .300 necked down cases (note which don't seem to grow at near the rate the 7mm cases do). Which raises a few more questions for me. I would think the shorter case lengths would have less neck tension thusly effecting POI and velocity to some degree. I haven't used a Chrony on any of my loads as I use the OCW method. Another worry question would be the effect of throat wear. Just the short amound of time and rounds used so far I do not notice any change in POI bases on case length: howver, COAL with the same charge quickly changes POI. It seems the 7MM cases quickly grow but the .300 necked down case do not have any appreciable growth. I don't believe in the life of the case they would ever require trimming to spec. length especially given my chamber size of 2.065.

Anyone else have any thoughts or experience with this case?

 

[Grimstod]

A reloading video that says about what is said above. Although it is for 7.62x54r it's the same principles.

[youtube]http://youtu.be/5qFHR-JUDNw[/youtube]

 

[expiper]

FD (frank) I like your homemade chamber guage,,,,very ingenius,,,,Roger

 

[dturnersr]

I agree with his thought and testing process in the video. However, right now I don't get the huge difference as he did with actual group size. I agree with his thought that the longer brass length is better for holding the bullet stable, but I still think that the shorter brass- .300 necked down to 7MM at about 2.05 length would have less tension thus higher velocity and different bullet strike or POI. As I said before out of curosity I will let the brass grow and retest with known group size. The bottom line I think the OP is trying to figure out is how the shorter brass effects accuracy. For me it's that and wear of the chamber. I like a bunch of people have limited 7MM Saum brass but have twice as much .300 Saum brass.

I would still like to hear from those that shoot a ton of 7mm saum.

 

[Grimstod]

Something that isn't mentioned in the video is that brass that is to short will leave a carbon ring in the chamber. Any form of build up in the chamber = inconsistencies.

"7MM at about 2.05 length would have less tension thus higher velocity and different bullet strike or POI"
Are you sure your not thinking the other way around? More neck tension should = more pressure thus more velocity. People that crimp everything (I am not one of those) claim to get greater MV with more tension provided by crimping.

 

[mikecr]

Since your necks already have clearance I would wait until they are stable in dimension(fully fireformed) before final trimming, provided some clearance always exists.
As long as you mind your HS, you can get pretty close to chamber end. I set -5thou as a controlled standard because I like the instant sealing/lower ES provided. Many folks think excess clearance doesn't matter, but as with all things reloading -it depends.

 

[fdshuster]

Roger: I cannot take any credit for the idea: got it from someone else's posting on this site. I had always used the dimensions from my reamer print drawings or those with the Sinclair gauge.

When I saw this, tried it more to find how close the results would be to a length taken with a Sinclair gauge, for 308. Both methods gave the exact length, 2.035".

A good alternative, especially if the gauge is the only item you "need" from Sinclair. Shipping charges will be more than the cost of the gauge.