determining jam length?

[lazyeiger]

After reading Tony Boyer's book I decided to determine the jam length of my setup. I am getting 0.015" difference in jam lengths with 10 trys ( different bullets and cases) I am measuring to the ogive. Thinking this may be a neck tension issue I did the same test using new unfired cases and I get the same 0.015" spread. What am I doing wrong?

Thanks Jon

 

[BoydAllen]

Jam should be measured with the same neck tension that you plan on using for your loads. This should be done with a case that has been fired and sized a few times. Another thing, no benchrest shooter that I am aware of cleans the inside of his necks so that they are free of powder fouling. They get more consistent seating by leaving some in the neck. My procedure is to use a retired bore brush for cleaning out necks, and to simply push in into case necks till it bottoms, and pull it out, doing so about three times per case, rapidly. If your bullet pull is consistent, your jam will be more so. Also, do not expect jam to be the same for different bullets. Their differences in shape mean that the distance form where you are measuring, and where the bullet touches the rifling are not the same. Another thing to keep in mind is that different bullet shapes seem to like different seating depths, what works well for one, will not for another. I have found that double radius bullets shoot better near jam, with long marks, and that more conventional shapes seem to like marks that are shorter than wide. Using this, I have been able to get to a load quite a bit faster.

 

[lazyeiger]

Yes these were 4 times fired cases but they had been cleaned with metal media so the inside of the necks were squeaky clean. I tried smooth cote on the inside of the necks and this reduced the friction ( gave a shorter jam length ) but the spread was still 0.015". Interesting that the relative jam lengths remained the same with or without the smooth cote.

 

[artbosco]

After reading Tony Boyer's book I decided to determine the jam length of my setup. I am getting 0.015" difference in jam lengths with 10 tries ( different bullets and cases) I am measuring to the ogive. Thinking this may be a neck tension issue I did the same test using new unfired cases and I get the same 0.015" spread. What am I doing wrong?

Nothing.

Each cartridge: case/bullet combination is its own species. Dimensions are going to vary. Copper is a very soft material so even a slightly different amount of exertion/force by you, is going to be picked up by your measuring tool and give you different readings.

Do you know how much force is required by you to go from Zero to 5/1000"? To go from Zero to 15/1000"? I don't myself.

That's why for my 6PPC and 30BR I look for visual confirmation of either a SQUARE mark on the ogive or a mark TWICE as long as it's wide, for my jam lengths. This whole process is very subjective.

 

[expiper]

listen to the advice ....this is something that can NOT be measured..(ooo yea it can...buttt) it should be visual (how the marks look) and by feel when closing the bolt.....what you feel or dont feel will tell you when its rite...throw away those stony point gadgets and meauring nuts....your chamber is what they gota fit...Roger

 

[lazyeiger]

I hear you all about this being a subjective proceedure, I just like to measure things, it's in my blood!!

Anyway I took some advice from Boyd and did exactly the same measurement with fired uncleaned cases and guess what now the spread is only 0.002" I aslo did the same meaurement with squeaky clean cases but very lightly lubed the bullet with Imperial sizing wax and these gave a total of 0.003" spread. Is there any reason not to lube the bullet?

It seems to me that if seating depth is one of the most important factors for accuracy then there must be a way to seat the bullet to exactly the same depth everytime. Of course this is only important if the bullet needs to be seated into the lands.

Jon

 

[BoydAllen]

One more thing, no one in benchrest (that consistently does well), that I am aware of, does the kind of cleaning that you described. They leave some fouling in their necks for a reason. It works better. If you have a thing about cleaning, express it by doing something else. Benchrest is all about what works. If you want points for appearance, build a show car.

 

[Tozguy]

Jon, I have a suggestion. Take 5 cases as you are going to shoot them but don't load them (no primer). Adjust your seater to the shortest jam length you have found and seat a bullet in each of the five cases. They should all be exactly the same length from the ogive by whatever measuring device you are using. If not, your measuring stuff might be the problem. Then chamber the five rounds and remeasure. If any come out longer its because the bullet is sticking in the lands. If they all have marks from each land and are the same oal as when you began then you have your jam length. If any are longer or shorter after chambering, repeat this test at the shorter setting.

 

[BoydAllen]

Jam is just a starting point. Once you record the measurement off the ogive you have something that is reproducible that you can either use as is (Be prepared to stick a bullet and dump some powder in the action at the time that is least convenient.) ,or something shorter. These days, for my short range benchrest rifles, I mostly go by rifling marks, but of course this would not work if I were jumping bullets to the rifling, which some good shooters do (a minority).