So I figured out where my lands are yesterday with my freshly upgraded rifle. I used a dummy round with a 168g H AMAX that had the ojive area colored with a black sharpie. I found out the exact length to have this bullet just kissing the lands and measured it from ojive to base with my digital caliper and a comparator. Can I use this same measurement when assembling cartridges with other bullets as long as I measure from ojive to base? It seems to me that I can.
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Curious question about measuring length from ojive
- Thread starter SWThomas
- Start date
Different bullets will have different oGives, so you have to measure each brand and model of bullet accordingly.
I realize the ojives will be different. But doesn't the comparator measure at the ojive of the bullet? If so, wouldn't the measurements be the same? As I understand it, the ojive is what contacts the lands first when the bullet is fired, and the comparator is sized to measure at the ojive when a bullet is inserted. So in theory, I should be able to use the same base to ojive measurements no matter what bullet I use as long as I use the same comparator to measure them? Right?
So if the oGive is different (see how I'm spelling it correctly there?) how could the measurement be the same? Bullet lengths vary as does the position of the ogive. If all ogives were standard then there would be only one for each bullet caliber, i.e. all 308 BTHP would all have the same ogive but they do not.
Because I'm not measuring just the bullet. I'm saying to measure an assembled round from the ogive to the base of the case. Take that measurement and seat a different bullet in a case until you get the same ogive to base measurement. If using the same comparator gauge and both bullets are of the same caliber, they should both be the same distance from the lands.
"Ogive" just means the pointy area in the front of the bullet. It could be a cone, a spherical shape, or something in between. I think this confuses people - you cannot "measure to the ogive" - you can only measure to a point on the ogive defined by the diameter of the comparator. This measurement, combined with the shape of the ogive determines how far from the lands it will be, not just the diameter of the comparator. You cannot compare different shaped bullets and expect them to line up with the lands the same, even if they are the same caliber.
damoncali;3112872[B said:
wrong poit of the bullet is the Meplat..... oGive is the curved part of the bullet.
Simply put.....you want to shoot 175smk say. Take one and measure chamber to lands. Then you can use that reading to seat those same bullets at a specified depth per your comparator. Once that lot number of those bullets are done....repeat step 1 with different lot # of same bullets or different bullets all together. Kind of understand now?
I had this same question a while ago and went through the same thing thinking that because I was using the comparator to measure the ogive (which I thought was the exact spot on the bullet where it would contact the lands of the barrel) that as long as I used the comparator that I would be measuring to that same diameter on each bullet no matter which shape they were, and therefore could use the same measurement... Which I did, and it only took one loaded round of the new load I was starting with 52gr smk to visually notice that something was off compared to the 55gr spire points I had took my original measurement with. I stopped working on the load and did some research and talked to a few people and actually tried to argue my point to several guys with decades more experience than me who had simply said "no you have to measure each type of bullet". Finally someone explained that the comparators were not a specific diameter, they were close but if you were to take several different brands of comparators and use each one to measure the "ogive" of the same round each one would measure slightly different. The comparator is not used to measure the exact beginning of the ogive but rather to give a consistent point to measure at along the ogive so that you can relate that measurement to your cartridges overall length at which that particular style of bullet is contacting that particular barrel. If any of this is wrong someone please correct me because like I said I had trouble with this for a while also.
Also if anyone could verify this id appreciate it. Say you insert your OAL tool into the chamber with your 308 casing and a 155 smk and seat it out to the lands like normal, but when using the comparator to measure it instead of the .30 cal bushing that is needed that would seat further back on the bullet close to the beginning of the ogive at nearly .30, say I used a .22 cal bushing. I know this is no longer measuring near the beginning of the ogive where the bullet would be contacting the lands and therefore would cause huge measurement differences between that 155 smk and say something like a 190 RN but as long as the .22 bushing was used when checking OAL of your loaded rounds also, would it not be essentially taking the same measurement but just using a different reference point. I ask because that is how I kinda had to explain it to myself to understand that the comparator was not measuring where the bullet actually contacted the barrel but was only giving a constant point to measure against. Thanks in advance for any feedback and hopefully I'm not still wrong and further confusing anyone.
Also if anyone could verify this id appreciate it. Say you insert your OAL tool into the chamber with your 308 casing and a 155 smk and seat it out to the lands like normal, but when using the comparator to measure it instead of the .30 cal bushing that is needed that would seat further back on the bullet close to the beginning of the ogive at nearly .30, say I used a .22 cal bushing. I know this is no longer measuring near the beginning of the ogive where the bullet would be contacting the lands and therefore would cause huge measurement differences between that 155 smk and say something like a 190 RN but as long as the .22 bushing was used when checking OAL of your loaded rounds also, would it not be essentially taking the same measurement but just using a different reference point. I ask because that is how I kinda had to explain it to myself to understand that the comparator was not measuring where the bullet actually contacted the barrel but was only giving a constant point to measure against. Thanks in advance for any feedback and hopefully I'm not still wrong and further confusing anyone.
Consider testing your own theory. It shouldn't be too difficult or time-consuming to compare the results between two methods of measurement with a few different styles of bullets.
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I sure will when I get time. My current job has pretty much haulted my reloading and shooting. I make it home maby twice a month for a day or two at a time and with everything else I usually don't make it to the reloading bench. But ive been down here almost a year now so hopefully soon I get back home and can get back to working on some new loads. It's pretty disappointing to finally make it back with an evening to reload and realize you don't understand a measurement and quit loading for that reason. But I did get a few of my old load cranked out and unfortunately that's all iv even had time to shoot since then. Just thought maby someone could back up or disprove my theory, but once I get some time I'll be sure to check it out and post back here. Sure seems to make sense to Me but iv been wrong before
No problem like I said I really had a hard time understanding it after several people explained it to me as the comparator measured from right where the bullet touched the barrel. It just didn't make sense that I should have to measure each style of bullet but that's defiantly the case. Glad it help you make sense of it.
If you had a piece of your barrel cut off to use as a comparator it would probably work as you asked. Am I wrong in this assumption?
Nope that's how I look at it too... Untill the throat starts wearing out. But as far as that goes if you had a bushing cut to the same diameter of your lands it would be fine but that's kinda besides the point, as long as you realize your not measuring where the bullet touches the lands, but only trying to keep a consistent point to measure from you should be find as long as you measure each brand/style/lot number.
Colt your on point a little bit....the comparator bushing should be the relative size of the common rifling ID of that caliber. A 308 has an absolute ID of .300 where the bore, or groove ID is .308. If you wanted to be that technical, it realign depends on leade angle and type of oGive of the bullet. With the different angles of each, the bullet comes in actual contact of the rifling at different points. An Amax will contact the rifling at a different point than will the vld. In other words, on may contact rifling at a diameter of .304 whereas the other will make contact at exactly the .300 mark. All depends on what angle the chamber is cut and type of bullet
The comparator set gives you a common measurement to go by no matter where the actual contact on the lands are. It will still be as accurate in seating depth off the contact point
Thank you all for making this very clear. I have a question about throat erosion regarding seating depth. I was told that the barrel will have a longer life if you are using a bullet with a secant ogive, like a VLD compared to non VLD, if the VLD is seated at or into the lands. The theory being that there will be less ignition on the throat and therefore cause less erosion. Does this sound right?
Thanks
Thanks
I understand the ID measurement and ID of the groves but I am not familiar with the term leade angle. Just guessing but it seems that the lands are cut at an angle, coming out of the chamber at the .308 groove ID and tapering to the absolute ID. Is this correct? If so that would better explain the different types of ogives contacting at different points.
Thanks for bringing that up again I think that's really the most important part to remember for anyone who's having difficulty understanding why each different bullet has to be measured
To my knowledge not true. Throat erosion is caused primarily by powder burn. Someone else may chime in with better info but in my years of firearms and reloading knowledge it really has nothing to do with type of copper jacketed bullets. My ultra mag will eat a barrel in 1200-1500 rds whether I'm using 200vlds or 230 hybrids with jump. Amount of powder and type used are the biggest equation
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Colt250....you are understanding it correctly. Comparator is used as a common point on a bullet for judging seating depths, not as a specific point on the bullet where it actually contacts the lands. In theory you could use a .25 caliber comparator on a .30 cal bullet and it would still work the same. As long as you take the measurement from the same place on the bullet when it makes contact in chamber and where your setting your seating depth from....
^^^ This^^^
And this is why different bullets will give you different readings. I've measured 5-6 different bullet types and got just as many differences in readings. Anywhere from 2.209" to 2.226". All because of the different shape of the bullet.
Also know that different comparators will contact the ogive at different places. For instance, the Sinclair comparator will contact further down the bearing surface than the Hornady comparator.
This is why they are COMPARATORS and not absolute measurements.
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