To put your query on Mayflower's guns in some sort of context, the following, by Dr R C Anderson is relevant "When I was asked to prepare a design for a model of the Mayflower, I was obliged to say that, to the best of my knowledge, such a thing was impossible - although I could produce a model of a ship about the right size and type." Of the guns, he writes "I gave the model twelve guns, eight minions and four sakers, and I was glad to find that the actual contemporary guns, which are now at Plymouth, Massachusetts, agreed very well with the shape of the guns in the model." If true and their provenance is established beyond doubt, then there is the answer to your query - either visit or contact the Plimouth Plantation, 137 Warren Avenue, Plymouth, MA 02360 USA. The ship was, incidentally, apparently very old in 1620; Dr Anderson speculates that it may have been part of the English merchant fleet in 1588! His model is that of a ship c.1600.
There follows a list of references, of which you may wish to take advantage:
a. Mariner's Mirror, Vol.XII, No.3, July 1926, which contains Dr Anderson's article.
b. "The Mayflower and other Colonial vessels" by Wm Avery Baker, who designed the replica which was sailed across the Atlantic to Plymouth in 1957. This vessel was built in Brixham, Devon from July 1955 and launched in September, 1956. The book contains much that is of interest - a glossary of terms, description of early ship design and building, rigging with description and sizes, mast and spar dimensions and research sources. There is also a page headed "Plans for Modellers" shewing Mayflower II on four sets of plans to 1/8in. scale. There are also plans for other craft of the period. Conway Maritime 1983, ISBN 0 85177 286 2.
c. "Give me a Ship to sail" by A Villiers, who captained the voyage in 1957 - a seaman's point of view, with several photographs. Published by Hodder and Stoughton, London 1958.
d. Plans Ref. MM444 (2 Sheets with a 12 page leaflet on the construction of a 1/72 scale model of the ship, solid up to the lower deck.) by R J Collins, who made his hull a trifle more full and with a little more planksheer than usual - in view of the vessel's age, i.e. a vessel of c.1580. He also speculates that the guns may have been carried as part of the ballast. I wonder if the guns were intended for defence of the settlement, rather than defence at sea. Given the likely age of the vessel one wonders whether the recoil of such weapons (though relatively small) in a sea fight, might not have done more damage to the Mayflower than to the enemy! Try www.Modelboats.uk or write to Magicalia Publishers Ltd, Berwick House, 8-10 Knoll Rise, Orpington, Kent BR6 0EL, England. I hope this helps. Good luck. Flitch.