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The problem "Flat based Vs Boat tailed" in LEs is due to a misconception derived from the introduction of the mark VIII (8) bullet in the late 1930s, which was specifically designed for Long range MG use ( as had been developed by other countries as well). It was found that Bore Erosion patterns were differewnt between Cartridges loaded with Cordite and those loaded with Nitro Powder..so much so that Vickers guns which had been fired with a lot of Mark VII Cordite were dangerous to use with Mark VIII ammo, due to bullet inaccuracy caused by the Bore erosion...this was especially so with "Overhead Fire" applications. ( erratic Bullet flight could and did cause Fatalities during training and on the Battlefield to "friendly forces".)
An order was issued that Barrels should be marked "for Mark VII only or for Mark VIII only" when packing barrels in Vickers Guns CES.
The major reason for this problem is "Base Upset Obturation", a principle known since the Cast lead Minie' Bullet days of Muzzle loaders, and utilised during the short period of use of Jacketed Small calibre Round nose Cylindrical bullets...an "under-Groove diameter RN proj. was "bumped up" by the base expansion into the full depth of the rifling groove, giving a "driving band" effect and sealing even a Worn or Eroded Bore, thus giving good accuracy with FB bullets. THis carried over to Pointed bullets with Flat bases.
But when Boat-tailed bullets came ito use for Longer range shooting ( around WW I,) it was found that accuracy dropped off as the barrel got worn or eroded...because a Boat-tail does NOT "base-obturate" sufficiently (if at all) to overcome wear and erosion.
The Austrians found this when they developed ( with Swiss Solothurn co-operation) the M30 8x56R cartridge for replacing the older M95 ( actually M93) Austro-Hungarian 8x50R cartridge, which had relied on Base-Upset-Obturation, as it was a Cylindrical RN Bullet...They had to make the Boat tail .329" diameter, the same as the Rifling Groove diameter, to avoid inaccuracy. The original 244 grain Cyl.RN, was .324 diameter, with a slightly cupped base to assist "upset".
SO these days, with Milsurp Lee Enfields (Mostly SMLEs and No4s) which in their service life used Mostly (or exclusively) Cordite based Mark VII ammo, the bore wear pattern will be that of Cordite. The continued use of either Cordite Milsurp Mark VII ( getting scarcer by the day) or its substitute, Mark VIIz loaded with Nitro Powder, (Canadian DIz or Yugo 1980s mil. ammo) will not cause any drop-off in accuracy...but the use of Modern (ie, commercial) Boat tailed projectiles in a .303 rifle will see accuracy problems ( Unless the rifle is an FTR Aussie Lithgow, with a "new" barrel ( ie, without a Cordite Wear Pattern); then the BT will function quite well in a "New" (Little shot) barrel.).
I use both FB and BT bullets in my Ross Mark III ( M1910) Chilean Navy Rifle, but it has a virtually NEW Barrel (made late 1916, and probably little used, even though it has a LC ( Large or lengthened Chamber) Mark, when all the rifles in service on the Western front were withdrawn (1916) from service and allocated to "Other" non-intensive uses (Training, Royal Navy, Tsarist military aid, etc).
The Chilean-marked Ross was part of the "Ships Arms Locker" of the Former HMS Canada, a Cruiser sequestered at the Makers in 1914, from the Chilean Gov't; it was returned to Chile after the war, and served in the Armada de Chile until the 1950s or so. ( Originally it had been equipped with M1912 Steyr Mausers in 7mm...these were allocated to Minesweepers for "Mine sinking" and 7mm ammo supplied by Kynoch and Eley in classic "Gov't " wrappers).
I have found that the Ross, (with its "newish" barrel, shoots just like a "New" Aussie FTR SMLE) but since I don't use Cordite Mark VII in the Ross,
only haldloaded PPYU berdan cases and either FB Mark VII projectiles or PPU new BT projectiles, I haven't seen any drop off in accuarcy ( at least not due to Bore wear.)
The main baseline to this problem is:- If your LE/SMLE/No4 is a worn barrel job ( Cordite) then stick to Flat based Bullets in Handloads and Milsurp.
If it is a "New" barrel Rifle, then either will work, or you can use BT handloads.
Regards,
Doc AV