Texas 10 is correct. The 53 SMK #1400 flat base bullet is very accurate in my .223s as is the Berger 52 grain
#22408 flat base.
I have heard that flat base bullet are accurate for distances under 300 yards but most of my shooting is at 100 yards at our range so they perform very well in my .223s, even the 1:9 and 1:8 twist barrels.
We are apparently using two different measurements so that may be the reason for the confusion.
I am using Overall average length (O.A.L) - measured from brass base to bullet tip, brass base to ogive length, overall average bullet length base to tip, bullet base to ogive length, and trim case length.
Most manuals provide Overall Average Length O.A.L. - bullet base to tip as their recommended seating. Sierra manuals use that length.
I measure my chambers using a Midway chamber depth gage that measures brass base to ogive to allow you to measure chamber depth directly.
You do have to be careful not to drive the bullet deep into the rifling or the chamber measures come up shorter than actual.
I remeasured the 52 and 69 SMKs with the Hornady gage I use to measure my chamber depth and got a slightly different bullet length for the base to ogive.
My last measurement were with a multi-caliber ogive length gage that apparently is slightly tighter and overstates the base to ogive length.
The difference in base to ogive length dropped from 0.332 to 0.256 with the 52 SMKs and from 0.480 to 0.395 with the 69 SMKs.
The thing that is confusing me is that chamber depth has to remain the same regardless of bullet type, assuming the bullet ogive is not being forced into the rifling to different depths. The measurements you have provided would indicate something is changing.
The thing that determines where the bullet can be seated is the position of the ogive since that is where the bullet tip finally increased to the full body diameter.
That is where the rifling can engage the bullet so if you are getting marks on the rifling, the ogive is what is showing the marks.
You can't adjust the steel in the barrel, although you can erode the rifling steel over time, especially if you load near Pmax.
The position of the ogive can be calculated and because it is in a fixed relationship to the bullet tip (+/- a few thousandths for BTHP formed bullets like the SMKs) the base to ogive measurement can be determined from the measured O.A.L.
Using the new bullet base to ogive measurements:
A 52 grain bullet seated at 2.250 O.A.L. would need a nominal chamber depth of 1.811 to just touch the rifling (a jump of 0.000).
A 52 grain bullet seated at 2.200 O.A.L. just touching the rifling would need a nominal chamber depth of 1.761. That is just 0.011 longer than the case trim.
A 69 grain bullet seated with 0.000 jump at 2.265 would have to have a chamber depth of 1.773 so we have to conclude that your chamber is at least that deep.
All of my .223 chambers are deeper than 1.800 and I regularly seat my 52 SMKs at 2.270 O.A.L.
If you can load the 69 SMK out to say 2.265 O.A.L. just touching the rifling, your chamber should be 1.778 to the rifling.
That chamber depth is still a lot shorter than any .223 chamber I have measured.
That would mean you should be able to load the 52 SMK out to 2.217 O.A.L. before they touch the rifling.
When seated at 2.200, the 52 SMK would have only 0.011 of the body available to get marks on the bullet body and the chamber would be 1.761 deep if there weren't marks.
I would think that the neck of the brass, being wider than the bullet. would be touching the chamber before that last 1/100th of an inch of the bullet body was touching the rifling.
If you are seeing marks on the 52 SMK bullet, then the bullet would have to be seated out much further than 2.200.
The 69 SMK, when seated at 2.260 in a 1.761 chamber, would have only 0.023 of the bullet body visible and the body would be 0.021 deep into the rifling if the chamber were only 1.837 deep.
But you are finding no rifling marks on the 69 SMKs at 2.260 so the chamber has to be at least 1.773 deep to not mark the 69 SMK ogive.
A 1.773 chamber depth should let you seat a 52 SMK to at least 2.212 O.A.L and allowing you to have the ogive only 0.023 out of the neck.
That's why I think there is something that doesn't compute.
There seems to be a difference between what you are measuring with the 52 SMKs and 69 SMKs and what I would conclude would be the chamber depth based upon the 69 grain SMK.
The 52 SMKs aren't very long bullets and with their boat tail they have an even shorter bullet body touching the neck.
I have heard that non-concentric seating is possible with some types of dies but I have never experienced it myself.
Is it possible that the 52 grain bullets are being seated askew slightly so the bullet is touching the rifling on only one side?
Are the rifling marks on the bullet consistent around the bullet body on the 52 SMKs?
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