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Measuring chamber length
Posted: Tue Apr 17, 2012 9:42 am
by Hauptman
I hope this is the right section for this topic.
I took a once fired 308 case, cut a slot 1mm wide down the neck to the shoulder with a Dremel. I placed a SMK2155 bullet in the case, chambered to round carefully in the rifle that the case was from, extracted, and measured the cartridge length to the ogive. I repeated this five times and got an identical reading each time.
I then repeated the exercise with a SMK2156 bullet, once again getting five identical readings, but 0.42mm (16 thou) longer than the 2155.
The 2155 bullet gave a length of 57.57mm, and the 2156 bullet gave 57.99mm...................
Am I missing something, because I would have thought the readings should be the same for both bullets?. The 2156 is longer than the 2155, but surely that shouldn't make any difference when measuring to the ogive.
All comments welcome..............................constructive ones, that is
)
Pete
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 10:35 am
by 20series
Pete
The bullets are a different shape, where the bullet engages the lands of the rifling is called the ogive. this is the best place to measure as it is generally more consistnat than the tips as they are sometime uneven.
Hth
Alan
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 10:50 am
by Watcher
Alan,
If the tips are uneven won't this be reflected in the ogive?
M
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 10:54 am
by phaedra1106
He does say he's measuring to the ogive hence the lengths he's quoted. As you say the profile of each bullet will be slightly different accounting for the difference in the measurements.
Watcher, no, tips won't affect the ogive measurement only the COAL.
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 10:58 am
by ovenpaa
Watcher wrote:Alan,
If the tips are uneven won't this be reflected in the ogive?
The easy answer is no, the more consistent measurement is from the Ogive (Curved section) as opposed to the Meplat (Pointy bit) which can vary.
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 11:17 am
by Watcher
ovenpaa wrote:Watcher wrote:Alan,
If the tips are uneven won't this be reflected in the ogive?
The easy answer is no, the more consistent measurement is from the Ogive (Curved section) as opposed to the Meplat (Pointy bit) which can vary.
Thanks. Short of buying a fancy tool (which I can't afford at the mo) how do I measure to the ogive? I'm about to do what Pete's talking about and must admit I was just going to meaure to the meplat.
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 11:23 am
by 20series
Many apologies for not reading the OP properly sign01
Alan
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 11:58 am
by ovenpaa
Watcher, do you have a vernier calliper?
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 12:40 pm
by Watcher
I do and a large magnifying glass to help me read it! Thinking about it do I just set the bore size and slide it down the bullet?
Re: Measuring chamber length
Posted: Tue Apr 17, 2012 3:35 pm
by Hauptman
Here's how I measure it. My assumption is that the diameter of the 2155 and 2156 bullets must be different at this point (ogive), otherwise the readings should be identical.
The parallel portion of the bullet starts to reduce towards the pointy end and the rifling contacts the bullet just after this point. Could it depend on the radius of curvature here? This radius will vary from one bullet pattern to another. So by using the ogive measuring attachment, it's not unreasonable to expect a small difference between bullet profiles.
I'm thinking that maybe I'll just set it up to the same ogive oal as RWS factory 308's and live with the 60 thou jump.
Thanks to all who have, and will, reply.