Something went disastrously wrong with my computer. I lost all that I had intended to post and there is now a huge gap between Big Jake's question and my answer for which I apologise. Here's my second attempt.
To give context to your question, R C Anderson wrote "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 knowlege, such a thing was impossible - although I could produce a model of a ship of about the right size and type." He continues "I gave the model 12 guns, eight minions and four sakers and was glad to find that the actual contemporary guns, which are now at Plymouth, agreed very well with the shape of the guns in the model." (Note a).
The earliest guns were of iron. The Anthony Anthony Roll 1546 (Note b) records 3680 iron guns against 471 of "brass", according to Caruana (Note c). By 1588 "brass" guns - introduced initially for breech-loaders, numbered 545, against 2 of iron (Note c); ten years later, of 931 guns - 137 were iron. Mayflower was an old vessel in 1620 and may have dated from the previous century (1588). IF the "actual" guns have a proven provenance, then there can be no argument, if not then it is possible that Dr. Anderson's mix of 4 Sakers and 8 Minions could, conceivably, be both iron and "brass" respectively, since I can find no mention of iron minions before the mid-17th century. In Note e, below, there is little if any, mention of the guns, which may lend support to R J Collins' view (Note f) that they were used as ballast.
Guns were of no consistent length or weight, both of which, using culverins as an example, could vary from 19-26 calibres (i.e. length times bore - 3"x19= 4ft 9") and from 46-54 cwt. In 1595 it is useful to note that demi-cannon of iron were lighter than those of "brass" (Note c)! Iron began to eclipse "brass" (c.1630) and by 1696, it predominated: during the changeover some "prestige" ships' armament was entirely "brass", e.g. Prince 1670 and the famous Sovereign 1637, (re-equipped entirely with iron in 1696).
Finally, the Establishment (e.g. 60 guns) of a vessel is no guide whatsoever to the number actually carried: HMS Prince 1670 was established for 100 guns; she actually carried 94.
Note a. Mariner's Mirror Vol.XII, No.3, July 1926. Model made for the Plimouth Plantation.
Note b. "The Anthony Anthony Roll of Henry VIII's Navy". Edited by C S Knighton and D M Loades, published by Ashgate for the Navy Records Society in 2000. ISBN 0 7546 0094 7.
Note c. "The History of English Sea Ordnance 1523-1875", Vol.I by Adrian B Caruana, published by Jean Boudriot Publications, England in 1994. ISBN 0-948864-20-6.
Note d. "The Mayflower and other Colonial vessels" by William Avery Baker, designer of the replica which was sailed across the Atlantic in 1957. It describes the design and building of the vessel and contains detailed plans, rigging details (with drawings and sizes) and a list headed "Plans for model builders", available from Plimouth Plantation, Inc. Box 1620, Plymouth, Mass., USA 02360 of whom it may be possible to secure details of the armament.
Note e. "Give me a Ship to Sail" by A Villiers. A detailed account of the 1957 voyage by the Captain, with several interesting photographs.
Note f. Notes and Plans by R J Collins who speculates that the guns may have used as ballast and not positioned on deck at all. Published by "Model Boats " magazine (England) as Plan MM444 . Hope this helps. Good luck. Flitch